Release RBTools 0.5.7.
Bump to 0.5.8 dev.
Fix the version number in the release notes.
Normalize path case names for perforce client path comparison.

When checking that the current working directory is within the perforce client,
we weren't normalizing the case of the paths. This was fine on case-sensitive
file systems, like Linux, but broke for Windows systems. Trivial fix.

Testing done: Ran unit tests

Fixes bug 3221.