Update travis badge in README.md
Update .travis.yml

This adds GHC 7.8.3 and GHC HEAD to the build-matrix, and removes a couple of redundant GHC versions.
Drop GHC 6.12.3 from build-matrix

It's starting to require too many workarounds...
Enable isRelativeDrive tests by moving them to isRelative

Delete only the following trivial test:

    isRelativeDrive "" == True
Fix links and add more tests and comments
Refactor: use hasTrailingPathSeparator
Refactor: joinDrive=combineAlways
Refactor: introduce function hasLeadingPathSeparator for clarity
Cleanup: rename file to dir in takeDirectory
Bug fix: pathSeparator can be forward slash on Windows
Refactor: simplify by calling dropTrailingPathSeparator
Refactor: propSep is not needed
Refactor: break instead of span and 3x reverse

This works because pathSeparators can only occur at the end of splitPath
elements.
Change: isDrive "" == False