Enable/disable services when connections go up/down

[adam-p]

The Tor service should only run when the device has connectivity. Peer pulling and Nominatim requests (etc.) should only be done when the Tor circuit is established.

Relatedly, Briar has added Tor-disabling-triggered-by-Android-Connectivity-Manager: http://sourceforge.net/p/briar/prototype/ci/c779d7b95a0af06f0b1977f2e5daee886bcfe42e/tree/briar-android/src/org/briarproject/plugins/tor/TorPlugin.java?diff=0cebc42ddbb5b6651651c6c3d84be48259410f48.

This is related to issue #37