Add caching and timeout to location fixes

NetworkGeolocationProvider.js provides caching and wifi scan timeouts.

http://hg.mozilla.org/mozilla-central/file/ac39fba33c6d/dom/system/NetworkGeolocationProvider.js

In the event that a wifi scan is started, but never returns with anything, a timer event should force a PositionWatch error.

In the event that scans are run too quickly, we should just return a cached position fix.
