pull-requests: fix problem with empty instances on creating PR from origin repo

tests: Use only one place to define pr and shadow URL segments.

tests: Add tests to check the shadow-repo regular expression.

shadow: Simplyfy the regular expression for detecting shadow repository URLs

shadow: Remove URL quoting of shadow repository pull URL.

tests: Adapt test to recent changes. This argument was removed.

tests: Fix tests which need the DB to run.

tests: Add a ``db`` fixture that initializes the database. This is quite useful if tests only need the database and not the whole app. Then only this fixture is needed instead of the full blown pylonsapp/app fixtures.

tests: Add tests to check the shadow repo exposure of the simplevcs class.

tests: Make return values of stub vcs controller editable.