add test for canonical_rev in BasicTest

This exposes a problem where GitRepo adds a newline to the end of the result.
SvnRepo and HgRepo pass the test.
strip output of GitRepo.canonical_rev

This removes a newline from the output.
Merge branch 'develop' into feature/fix-canonical-rev-newline
fix reference to unicode in SvnRepo.canonical_rev

Python 3 deprecates the need for 'unicode' so this causes a NameError when
it is referred to.
Merge pull request #36 from OEP/feature/fix-canonical-rev-newline

add tests and fix issues with canonical_rev()