[update] Send requested update interval to selected Geoclue provider.

Send the requested update interval to the active Geoclue provider. The
update interval is sent by passing and 'UpdateInterval' option to the
provider. Not all Geoclue providers support this setting, in which case
they will report position changes at their default rate.

The Geoclue position info source no longer limits the rate of position
updates internally.
Merge pull request #27 from amccarthy/master

[update] Send requested update interval to selected Geoclue provider.