Lazily choose whether to skip tests

This is the new and improved version of an earlier PR for lazily determining whether a given test should be skipped. (notice the branch name: `skiptests3`) The decorators `skip_if_bz_bug_open` and `skip_if_rm_bug_open` have been created to replace `bzbug` and `redminebug`, respectively.

Many thanks to @elyezer for help with the decorators.
Something like `skip_if_bug_open(type='bugzilla', id=42)`?