support cloning from ssh

[wryfi]

Trying to clone a mercurial repo via ssh fails as follows:

hg = MercurialRepository('/tmp/foo', create=True, src_url='ssh://hg@bitbucket.org/marcinkuzminski/vcs')
---------------------------------------------------------------------------
URLError                                  Traceback (most recent call last)
<ipython-input-55-bd9b5320f794> in <module>()
----> 1 hg = MercurialRepository('/tmp/foo', create=True, src_url='ssh://hg@bitbucket.org/marcinkuzminski/vcs')

/usr/local/lib/python2.7/dist-packages/vcs/backends/hg.py in __init__(self, repo_path, create, baseui, src_url, update_after_clone)
     87         self.baseui = baseui or ui.ui()
     88         # We've set path and ui, now we can set _repo itself
---> 89         self._repo = self._get_repo(create, src_url, update_after_clone)
     90 
     91     @property

/usr/local/lib/python2.7/dist-packages/vcs/backends/hg.py in _get_repo(self, create, src_url, update_after_clone)
    302                     opts.update({'noupdate': True})
    303                 try:
--> 304                     self._check_url(url)
    305                     clone(self.baseui, url, self.path, **opts)
    306 #                except urllib2.URLError:

/usr/local/lib/python2.7/dist-packages/vcs/backends/hg.py in _check_url(self, url)
    283         except Exception, e:
    284             # means it cannot be cloned
--> 285             raise urllib2.URLError(e)
    286 
    287     def _get_repo(self, create, src_url=None, update_after_clone=False):

URLError: <urlopen error <urlopen error unknown url type: ssh>>

Works fine for git (incidentally due to using subprocess module).