snf-ci: Test a pull request

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.