Transition to C++11 and CI jobs covering more cases

[piponazo]

In this first PR we finally do the migration to C++11.

Basically this PR is a continuation of #257 which was not merged in April 2018 because we took the decision of not switching to C++11 on the Exiv2 0.27 version.

I could not apply directly the commits that @D4N wrote in that PR, however all the kudos related to that change should be forwarded to him 😉 .

This PR solve some of the items discussed in #214 but not all of them. After this PR is merged, we can continue implementing some of the items that @D4N carefully wrote in that issue.

These are the changes implemented in the PR:

• Drop auto_ptr and replace it with unique_ptr
• Updates in Travis:
  • Switch from Ubuntu 14.04 to 16.04
  • Add new elements to the matrix. Now we compile for Debug and Release.
  • Two OSX jobs (with old and recent XCode versions)
  • Simplify definition of CMake variables.
  • Simplify conan management
  • Give names to each CI job to make easier its identification.
• Updates in Appveyor:
  • Drop old compiler (non c++11 compatible)
  • Expan the build matrix, so that we compile for all the combinations of Static/Shared,
    Release/Debug, X86/X64.
  • Simplify conan management
• CMake:
  • minimum required version updated to 3.5, so that we can use the new FindGTest IMPORTED targets, that are solving some problems I found during the addition of new jobs to the CI.
  • Remove many calls to check_include_file that were not needed. This speeds up considerably the CMake configuration step.
  • Project version updated to 0.27.99, to somehow indicate that the next version will be 0.28.
• Conan: Gtest updated to 1.8.1 (Fixes Update to gtest 1.8.1 #568)

Since the PR is quite large, I would recommend you to review it commit by commit. Each commit is focused in independent topics.

Note: I will squash some of the commits related with travis and appveyor. Right now there are many commits that I pushed while testing the proper execution of the CI jobs.

[piponazo]

Note that the Fedora job in gitlab failed due to some network issue, but the rest of the jobs passed.

[D4N]

I have restarted the Fedora job on Gitlab.

[codecov]

Codecov Report

Merging #601 into master will decrease coverage by 0.16%.
The diff coverage is 69.1%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
- Coverage   62.78%   62.61%   -0.17%     
==========================================
  Files         154      154              
  Lines       20704    21094     +390     
==========================================
+ Hits        12998    13209     +211     
- Misses       7706     7885     +179

Impacted Files	Coverage Δ
include/exiv2/metadatum.hpp	100% <ø> (ø)	⬆️
include/exiv2/types.hpp	96.42% <ø> (+0.27%)	⬆️
include/exiv2/psdimage.hpp	100% <ø> (ø)	⬆️
include/exiv2/webpimage.hpp	100% <ø> (ø)	⬆️
include/exiv2/properties.hpp	100% <ø> (ø)	⬆️
include/exiv2/matroskavideo.hpp	0% <ø> (ø)	⬆️
include/exiv2/cr2image.hpp	0% <ø> (ø)	⬆️
include/exiv2/xmp_exiv2.hpp	90.9% <ø> (-9.1%)	⬇️
include/exiv2/exif.hpp	100% <ø> (ø)	⬆️
src/version.cpp	89.65% <ø> (-1.15%)	⬇️
... and 171 more

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[D4N]

Thanks for all the work!