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).