Improve Opensocial HttpFetcher to user ProxySelector instead of Provider<Proxy>

Due to a shindig bug (http://issues.apache.org/jira/browse/SHINDIG-718), the proxy configuration is injected via Guice. This prevent using different proxy configuration based on the URI we want to fetch.

Solution :
Override the injected BasicHttpFetcher by a new one that is base on the java.net.ProxySelector method to select proxy.
The ProxySelector is injected by Guice the same way, but constructed by the OpenSocial Component