Make a suite daemon's cylc version discoverable.

This addresses https://github.com/cylc/cylc/issues/1243#issuecomment-64879494 (which is a minor addendum to #1243) by:

 * add cylc version to the suite environment file
 * a new function to the suite info interface, to return the daemon's cylc version
 * a new command "get-cylc-version" to call the new info function
 * and a corresponding test suite

@matthewrmshin - please review.