Added CreateServer to Region interface.

I did not add any tests for this API because it just re-uses
infrastructure which is already tested elsewhere.  The only differences
are the JSON request bodies and responses, which encoding/json handles
for us in a type-safe manner, and the specific endpoint and request
method, which Perigee handles for us.
Merge pull request #15 from racker/region-create-server-api

Added CreateServer to Region interface.