Replace +branchWithName: with -lookupBranch…

Also, change the method to use a _branch_ name instead of a _ref_ name
and use the simpler git_branch_lookup() API.
Tests for branch lookup
s/lookup/lookUp/
Merge remote-tracking branch 'origin/master' into branch-lookup
Merge remote-tracking branch 'origin/master' into branch-lookup
Add `success` parameter to -lookUpBranch…
s/lookup/lookUp/ once more
Merge pull request #314 from libgit2/branch-lookup

Move branch lookup to GTRepository and clean up its API