Remove useless method: "parent_ids"

Use "parent_oids" in ext instead of "parent_ids"
Bump libgit2
Fix the branch API to work with libgit2 upstream
Properly remove Unicode ref on teardown
Docstrings for Branch
Fix README.md
Merge pull request #101 from iblue/fix-readme

Fix README.md
Fix a reference to OBJ_* in the README.
Merge pull request #104 from nelhage/doc-fix

Fix a reference to OBJ_* in the README.
Merge branch 'development' into patch-3
Rakefile: introduce LIBGIT2_DEV env var to skip refreshing the submodule

We need this step in `rake compile` so that Bundler can install the
gem properly from git, but now we can disable the behavior for local
dev.
Bump libgit2 and update api usage
Update tests to use correct exceptions
Use Repository#rev_parse_oid in Repository#create_branch

This is the purpose-built API to lookup an arbitrary treeish, and since
git_lookup_reference() returns a new error code, it messed up the original two-stage
lookup.
Merge pull request #110 from libgit2/mayhem

Bump libgit2 version in Rugged
Merge pull request #100 from yandy/patch-3

Remove useless method: "parent_ids"
Add Rugged::Index#read_tree
Merge pull request #111 from libgit2/index_read_tree

Add Rugged::Index#read_tree
This is 0.17.0.b7 now
Update API usage for libgit2 1b93468

Updates usage of git_remote_download and git_index_read_tree.