Added a wrapper for GTFilter.
Wrap some stuff up.
Tests.
Unfit.
Doh
Unregister the filter after each test.
Add some more documentation.
Test the filter source that we're given.
Fix `userSignatureForNow` sometimes returning nil.

On the iOS simulator, `NSFullUserName()` (and presumably
`NSUserName()`) return an empty string; currently we check for nil.
This leads to `userSignatureForNow` returning nil and, eventually, a
crash. Instead make sure that `NSFullUserName()` and `NSUserName()`
don’t return an empty string or nil.
Errrryday I'm refactoring.
Test smudging
Use a simpler check.
It's == not !=.
Repeated calls are wasteful.
Merge pull request #328 from PiersonBro/fix-userSignatureForNow

Fix `userSignatureForNow` sometimes returning nil.
Expose the repository URL instead of the object itself.
Require an apply block.
+filterWithName: => +filterForName:
Merge pull request #327 from libgit2/filters

Filter support
Added -flushAttributesCache.