Adds toBeDate()

[thiagodp]

First step to #117

Housekeeping

• Unit tests
• Documentation is up to date
• No additional lint warnings
• Add yourself to contributors list (yarn contributor)

[codecov-io]

Codecov Report

Merging #118 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #118   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          88     90    +2     
  Lines         413    425   +12     
  Branches       69     71    +2     
=====================================
+ Hits          413    425   +12

Impacted Files	Coverage Δ
src/matchers/toBeDate/index.js	100% <100%> (ø)
src/matchers/toBeDate/predicate.js	100% <100%> (ø)

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[thiagodp]

BTW, Travis CI is passing for Node 8 but failing for Node 6.

[mattphillips]

This looks great @thiagodp thanks a lot 😄

[mattphillips]

I've published this in v0.7.0