Bug 811528 - pref native identity on; r=ferjm

https://bugzilla.mozilla.org/show_bug.cgi?id=811528

The native implementation of Persona needs to be enabled in b2g.

@ferjm points out that the proper place to pref identity on is on the gecko side: http://mxr.mozilla.org/mozilla-central/source/b2g/app/b2g.js

This PR is then only to enable identity debugging if `DEBUG=1`.  Merging this PR does not depend on landing the rest of Bug 811528 in gecko.