Add Eclipse proxy initialization to HttpClient instance

This commit actually refactors the way the Standalone Geppetto and
the Eclipse based Geppetto are assembled. Standalone and Eclipse
specific are moved into two separate jars:

org.eclipse.geppetto.injectable.eclipse
org.eclipse.geppetto.injectable.standalone

They both expose the package org.eclipse.geppetto.injectable which
in turn contains a CommonModuleProvider that has a static method
returning a Guice module containing some basic injections. One of
those injectios is the HttpClient binding which in case of Eclipse
is set up using the Eclipse proxy settings.