Add -[GTBlob applyFiltersForPath:error:]

Also included some methods for converting between `NSData` and `git_buf`, which is used extensively by libgit2 to represent chunks of data.

**To do:**

 - [x] Test the `NSData` category
> Should this have an error parameter?

The only possible failure is memory allocation, which Cocoa conventionally doesn't generate `NSError`s for.

> Should `buffer` be marked as `inout`?

`inout` is a relic of Distributed Objects. It doesn't mean anything at all.