Add methods to enumerate a GTTree.

Also adds `-contents` for quick access.
Log the error if the enumeration fails.
Add private methods to get a GTTreeEntry underlying object type

Also use that in `-description`.
Implement GTTreeEntry equality.
Make `-contents` not recursive.
Provide the correct parent to GTTreeEntry initializer.
Tests for `-contents`.
Document those.
Return a BOOL instead of the libgit2 error code.
Document the enumeration block return type.
Use `-stringByAppendingPathComponent:` here
Use object subscripting.
NULL pointer dereference.
Use dot-syntax.
Move private API in a separate file.
The enumeration block can't be nil.
Merge pull request #208 from tiennou/tree-contents

Give GTTree some love