Add Nokogiri to Required Gems

Update the gemspec to include Nokogiri for XML parsing. Also update the
main lib file to require the nokogiri libs.
Spec and Add Application ID/Sessions Path

Update the Version 11 spec and corresponding lib to include the path to
obtain the mappings of application IDs to corresponding active sessions.
Spec and Add application_ids_sessions for Version

Update the Version spec and lib to include a method that returns a hash
of application IDs to corresponding active sessions.
Spec and Add Version total_sessions_path

Update the Version spec and corresponding lib to include the
total_sessions_path (path to obtain high-level information about the
Prophecy instance).
Spec and Add total_sessions_mapping for Version

Update the Version spec and lib to include a total_sessions_mapping
method that maps different types of total sessions for the Prophecy
instance.
Fix total_sessions_path

Update the total_sessions_path to be the correct stats path.
Spec and Add total_sessions

Update the Instance spec and lib to include the total_sessions method
that returns a high-level snapshot of the current total sessions for the
Prophecy instance.
Spec and Add sessions_by_application_id

Update the Instance spec and lib to include a method that returns the
correpsonding sessions to application ID mappings.
Spec and Add sessions_for Method and Fix Specs

Update the specs to include the FakeWeb clean_registry to remove
existing URLs being faked, as well as spec and add a method to determine
the number of active sessions based on a specific application ID.