Allow bors to look at gihtub commit status API for merge commit

Pair-programmed with @reiddraper

Instead of polling buildbot's JSON status, if you have your CI server
(not just buildbot but even something like TravisCI) posting to the
github status API, you can use the new 'use_github_commit_status_api'
boolean option to check for the commit status on the merge commit to see
if the integration commit has passed the tests.
Support multiple repositories in bors.html

Pair-programmed with @Vagabond.

bors.html now allows you to choose a specific repository to view, based
on the query string. This is useful if you run bors for multiple repos,
but in the same directory. If there is only one repository, this change
is backward compatible, as it defaults to the singleton repository.
The main change is to wrap the json inside of a map, which is keyed off
of the repository name. The javascript that runs inside the html file
then looks up the queue, based on the query string. If no query-string
is provided (and there is more than one repository), then a list of
available repos is shown.
Make sure the PR links point to the destination repo
Merge pull request #26 from basho/check-github-status-api

Allow bors to look at gihtub commit status API for merge commit
Merge pull request #27 from basho/multiple-repo-html

Support multiple repos in bors.html