Bearer connection stats, updated to use rx-bytes and tx-bytes

[Aleksander Morgado]

This v2 series has the following updates:

 * Renamed the key names in the Stats dictionary to 'rx-bytes' and 'tx-bytes'.
 * Use G_SOURCE_CONTINUE update instead of TRUE as return for GSourceFunc (all the other GSourceFunc returns have already been changed in git master).
 * Fixed error reporting in the QMI bearer implementation, it wasn't properly finishing the context as soon as the error was found.
 * Stats reloading implemented in the MBIM bearer.

Comments/tests welcome.