Fix Ubuntu installation instructions.
Merge pull request #31 from c0d1f1ed/install

Fix Ubuntu installation instructions.
Avoid subprocess call for repository initialization.
Repo.branch(): Default to repo HEAD if no commit_sha was specified.
Simplify staging by calling out to dulwich convenience function.

Fixes some tests.
Cope with changes in branch_list() API
Pass DIR to Repo.status, as it is now mandatory.
Use dulwich.porcelain.tag for tagging.
Use dulwich.config.ConfigFile.
Remove custom config parser - call out to standard parser in dulwich instead.
Require newer version of Dulwich.
Simplify tag_list function.
Simplify Repo.commit by calling out to Dulwich.
Remove unused _get_config_user helper.
Simplify repo.fetch.
Remove unused exceptions.
Remove unused _get_working_tree.
Simplify Repo.export.
Avoid accessing private member variable of Index when removing entries.
Remove unused functions _write_packed_refs and get_sha1_from_refspec.