Geocoder http proxy support

With all of the geocoders using urllib2 for accessing the different geocoding web services, I have added http proxy support to the __Geocoder__ base class that these geocoders inherit from. 

This implementation uses the built in support for environment variables to be supported using the non defined __ProxyHandler__ class in urllib2. This allows for other protocols to be supported via '<protocol>_proxy' (eg. http_proxy) environment variables. 

Tests are included for this with a simple HTTP proxy.