Add find_requests service and requests API data

At the moment we don't do a great job when it comes to communicating
what's going on under the hood when you push a build to github - it
either shows on the builds list or not. In the latter case users may be
left in the dark, which I suspect may be more common in the future, when
we move such things like "exclude branch" to the repository settings
pane.

This commit reveals part of the request data, which we can use to show
what did we get from github and if it was accepted.
Raise when repository can't be found in find_requests
[find_requests service] Parametrize limit for recent requests
[find_requests service] Format dates in the same way as in other services
Include commits with requests in API
Add details about params to RepositoryNotFoundError
Add branch_name and tag_name to Request
Attach more info about requests in the API
Merge pull request #316 from travis-ci/requests-api

Add find_requests service and requests API data