Adds support for custom datarefs

Hi there,

i added support to export the variables "flying" and "leaseTime" via X-Plane's custom datarefs.

The following datarefs can be accessed from every other plugin within X-Plane:
"fse/status/flying"    can be 0/1
"fse/status/leasetime" is the remaining lease time

This script is running for month on my X-Plane 64bit (currently 10.30b5) under Windows 7/64bit without any problems until now.
I'm using FlyWithLua (2.2.4) to read the datarefs and do actions (play sound, pause X-Plane) depending on the exported datarefs and it works very very well!

I think it would be a great addition for the pilots at FSE, so they don't fly without start the flight in FSE, like here:
http://www.fseconomy.net/forum/fse-general-discussion/4616-what-is-the-longest-trip-without-hitting-start-flight
 :-)

An example script to display the current flight state with a FlyWithLua script
and some screenshots are attached.

It would be great, if this patch could be added to the official FSE script.

Cheers,
Teddii