Track/store if the user is available for sharing.

Just for Chrome so far.
Bundle description + sharing in GlobalSettings obj
Remove unused update functions.
Add button to toggle sharing state.
Merge branch 'dev' of https://github.com/uProxy/uproxy into lucyhe-add_stop_sharing_state
Send instance update when global settings update.
Allow user to set custom STUN server in UI.
Replace core.iceServers with core.webrtcPcConfig
Allow custom RtcNet configs + show confirmation.
Merge remote-tracking branch 'origin/dev' into lucyhe-stun_config_in_ui
Use same STUN server for socksToRtc and rtcToNet.
Add support for Polymer core elements.
Add core-icons to bower.json.
Allow user to reset STUN servers to default.
Change setStunServer logic.
Merge remote-tracking branch 'origin/dev' into lucyhe-stun_config_in_ui
Fix comments + remove unused code.
Merge pull request #619 from uProxy/dborkan-ff-fixed

Fixed polymer loading in Firefox
Add requesting/offering access roster groups.
Edit UI tests to account for two sets of contacts.