Add rdfReader files

[abhishekspeer]

RDF Parser support is added via this pull request.

[swinslow]

Hi @abhishekspeer, as we discussed you'll want to modify the import paths for some of the files so that they use github.com/spdx/tools-golang/ -- that appears to be why the CI checks are failing.

You may also want to consider changing the directory and package names from rdf to rdfloader in order to mirror how the tag-value packages are set up (tvloader).

[swinslow]

Hi @abhishekspeer, I am having some trouble pushing commits to amend the PR. The build is failing because Travis is failing to install libraptor, which is required for goraptor to work.

Could you please push another commit onto your branch that makes the following change?

In .travis.yml, please add the following after line 4:

addons:
  apt:
    packages:
      - libraptor2-dev

And then push that commit to your branch, which I hope will update this PR and re-run the tests successfully :)

[abhishekspeer]

@swinslow that was exactly where I was stuck! Thanks a lot for helping me with this one. :)
I've made the changes and all the checks have passed now.

[swinslow]

Great! I will go ahead and merge this into the rdfreader branch. Thanks @abhishekspeer!