Async http addition

* Addition of AsynchronousHTTPClient and AsyncResponse class

```
running nosetests
running egg_info
writing requirements to swaggerpy.egg-info/requires.txt
writing swaggerpy.egg-info/PKG-INFO
writing top-level names to swaggerpy.egg-info/top_level.txt
writing dependency_links to swaggerpy.egg-info/dependency_links.txt
writing entry points to swaggerpy.egg-info/entry_points.txt
writing requirements to swaggerpy.egg-info/requires.txt
writing swaggerpy.egg-info/PKG-INFO
writing top-level names to swaggerpy.egg-info/top_level.txt
writing dependency_links to swaggerpy.egg-info/dependency_links.txt
writing entry points to swaggerpy.egg-info/entry_points.txt
reading manifest file 'swaggerpy.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'swaggerpy.egg-info/SOURCES.txt'
/nail/home/prateek/pg/mypy/lib/python2.6/site-packages/twisted/internet/endpoints.py:30: DeprecationWarning: twisted.internet.interfaces.IStreamClientEndpointStringParser was deprecated in Twisted 14.0.0: This interface has been superseded by IStreamClientEndpointStringParserWithReactor.
  from twisted.internet.interfaces import (
/nail/home/prateek/pg/mypy/lib/python2.6/site-packages/twisted/internet/_sslverify.py:184: UserWarning: Your version of pyOpenSSL, 0.10, is out of date.  Please upgrade to at least 0.12 and install service_identity from <https://pypi.python.org/pypi/service_identity>. Without the service_identity module and a recent enough pyOpenSSL tosupport it, Twisted can perform only rudimentary TLS client hostnameverification.  Many valid certificate/hostname mappings may be rejected.
  verifyHostname, VerificationError = _selectVerifyImplementation()
................................................................................
----------------------------------------------------------------------
Ran 80 tests in 0.537s

OK
```
It feels weird to inherit from requests.Response here, because then we're tied to their implicit interface.  Is it possible to instead pull out our own interface than both AsyncResponse and requests.Response adhere to?