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twisted.internet.interfaces.IStreamClientEndpointStringParser was deprecated in Twisted 14.0.0: This interface has been superseded by IStreamClientEndpointStringParserWithReactor.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UserWarning: Your version of pyOpenSSL, 0.10, is out of date.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
```
How about putting all of the async classes in their own file?