Add 'system' channel to send 'pause' and 'resume' events to node.
Add 'events' to the channel to allow user defined events.
Allow user defined objects to be sent through the channel.

Add documentation for the channel API changes.

Adds app.datadir to the bridge to access a writable persistent path
from the nodejs-mobile runtime, which should be the FilesDir on
Android and the NSDocumentDirectory on iOS.
Also sets the os.tmpdir() to the CacheDir on Android.

Messages sent to the node runtime with values that evaluate as false
in conditions were reaching node as undefined value.
Fixes this behavior so that only undefined values can reach node.

Waits for the pause event handler to finish before letting the iOS
application suspend after going to the background.
Passes a pauseLock object to the pause event handlers to release so
the app channel can signal the application it can suspend.