Add GHCompare.getFiles() method to be able to see the precise files changed.

There is a file field inside of GHCompare but no getter to extract the values
for analysis.

There are contents in that field so I've added a new get method so that they
can be extracted.
Merge pull request #132 from mocleiri/add-getFiles-method-to-GHCompare

Add GHCompare.getFiles() method to be able to see the precise files chan...