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.
Interesting. You can connect to nonexistent URLs.

    >>> import bugzilla
    >>> URL = '13245'
    >>> bz_client = bugzilla.RHBugzilla()
    >>> bz_client.connect(URL)
    >>>