Refactor RemoteHTTPIndexedStore to not use urllib2._opener

enstaller.proxy.util.install_proxy_handlers installs a proxy handler
in the global opener, but the RemoteHTTPIndexedStore uses it's own
opener and this proxy handler also needs to be added to this handler.
We use a module level variable `enstaller.proxy.util.proxy_handler` to
holdon to the proxy handler and pass it to the opener used by
RemoteHTTPIndexedStore.