Add option '--pull-request' to snf-ci

This option gets a Github pull-request url (e.g. 'https://github.com/grnet/synnefo/pull/id') and runs the testsuite in a sophisticated way.

Sophisticated means that it will not just check the remote branch from which the pull request originated. Instead it will checkout the branch for which the pull request is indented (e.g. grnet:develop) and apply the pull request over it. This way it checks the pull request against the branch this pull request targets.