Extract update_repo method
Rename build_api_url to build_api_compare_url

We want to add a call to another API endpoint, so make the existing
method name a bit more specific.
Move api_compare_url and http_compare_url to object
Split repo update into an update and redraw method

As well as cleaning up the split between code which is working things
out based on the API response and code which is rendering stuff to the
view, this lays the groundwork for calling redraw independently of
updating the repo from an API call to the compare endpoint.
Merge pull request #5 from evilstreak/split-update-and-render

Split update and render