Add support for Git refs

[tchap]

Not much to say, http://developer.github.com/v3/git/refs/

I am already working on this and I will create a pull request once I push some tests.

I noticed that there is issue #51 open for tags support. We may define the same stucts, namepy GitObject, so I guess the one being merged later will have to modify his code...

[tchap]

Actually, I can see now that the roadmap document is not up to date. The support was added by the pull request, so Git Data > References should be saying Done except the sub-namespace support... Could someone update it please? @willnorris ?

[willnorris]

Updated spreadsheet. Though it does look like we're missing support for listing references in a sub-namespace (discussed here). Presumably that should be part of an Options struct passed to ListRefs.