Remove unexisting command repo.hash.
s/rugged/repo in Rugged::Commit section.
tag.target_type returns a symbol, not string.
s/rugged/repo in TreeBuilder section.
No method get_entry for index.
Fix Rugged::Reference part.

1. ref.type returns a symbol instead of string.
2. ref.set_target instead of ref.target=
3. Reference entry has keys id_old and id_new instead of oid_*
repo instead of @repo in Branch section.
Merge pull request #174 from DawidJanczak/readme_update

Readme update