Add file content matcher to FileMatchers

[night-gale]

Fixed #281 Add two public methods to FileMatchers. The matchesContentOf takes a File and returns a Matcher<File> that matches the content of the file. The aFileWithContent takes a Matcher<String> where the string is the expected file content, and returns a Matcher<File> matches the content of the file.

[nhojpatrick]

Going to try and kick start hamcrest, so if you want to get it merged, please rebase from the branch v2.3-candidates. Still trying to understand how has permissions to perform a release.