contents method added to Repo
Contents interface and Rt class added to API
Content interface added. RtContent and RtContentTest annotated with todo
puzzles
Content interface added. RtContent and RtContentTest annotated with todo
puzzles
Puzzle updated to reflect mock implementation needs.
formated MkRepo class.
Mocks for Contents and Content added.
Todo tests and integrations tests added to RtContents class
Code style issues fix.
Removed checkstyle check for ClassFanOutComplexity in class RtRepo
Merge branch 'master' of https://github.com/jcabi/jcabi-github into 118
Merge branch 'master' of https://github.com/jcabi/jcabi-github into 118

Conflicts:
	src/main/java/com/jcabi/github/Repo.java
	src/main/java/com/jcabi/github/RtRepo.java
Added puzzles for methods to implement in RtContents.
method name getReadme() changed to readme()
Added @Immutable annotations to Contents and Content interfaces
Merge pull request #175 from andrescandal/118

#175: pull request Issue #118 Repo.contents()  implemented