Proxy support for the agent

If the forwarder is installed, it uses tornado's CurlAsyncHttpClient
CurlAsyncHttpClient requires PyCurl which is not easily installable from source.
Therefore proxy support won't be working on source installation of the agent

If the forwarder is not used it uses urllib2 in the emitter.
Warning: 
* urllib2 has a bug on python < 2.6.3, this is why we bundle a patched version of urllib2 and httplib that will be used in case of connection through a proxy with python 2.6.3
* I added another patch to handle proxy authentication for SSL connections

It has been tested on:
- Windows
- CentOS 5
- CentOS 6
- Ubuntu 10.04
- Ubuntu 12.04 


DogstatsD doens't implement proxy support as it's local traffic in most cases.