Support descriptor creation

We've always supported descriptor creation in our tests but this turns out to
be a capability others want too...

  https://trac.torproject.org/projects/tor/ticket/10227

This is done via a simple create() and content() static method on Descriptor
classes...

  RelayDescriptor.create({'router': 'demo 127.0.0.1 80 0 0'})

Isis made her own library for this, which unlike us supports signing
descriptors. Definitely a capability we should incorporate. :P

  https://pypi.python.org/pypi/leekspin
  https://gitweb.torproject.org/user/isis/leekspin.git/