Rewrite branch URLs inside issue content

[philipstarkey]

Currently, URLs that point to branches link to the BitBucket archive (which doesn't yet exist, see #19) and doesn't also include a link directly to the list of commits for the branch on GitHub.

We should rewrite the URL so that it does both (much like we do for links to individual commits). We should see if there is autolinking syntax on GitHub for branches or if we have to construct the markdown URL ourselves.

This is also made more complicated by the fact that I can't find clear guidelines on what characters are allowed in a branch name, which makes matching the URL difficult.