Fix error code/domain when initing GTRepository with bad URL

Previous error used a code from kCFErrorDomainCFNetwork, but constructed
the error in NSCocoaErrorDomain.
Add method to initialize bare repositories

Also report an error if the passed URL is bad.
Add test for init bare repository
Convert repository initialization tests to Expecta
Merge pull request #228 from jmah/init-bare

Add method to initialize bare repositories