Add GTRepository+URLRelativeization.h
Merge pull request #13 from PiersonBro/repository-level-relative-urlization

Add GTRepository+URLRelativeization.h
Merge branch 'master' into refactor-commit
Make GTRepository+URLRelativeization.m more resilient.
Rewrite the committing system to use the index.

As part of the change:
- Removes IDTTreeWriter as it’s not needed.
- Renames IDTTreeCreator to GTRepository+TreeCreation.
- Update the app and tests.