Add ENV variable to control XulRunner Init

There still seems to be issues with WeSay crashing when XulRunner
is initialized.  It seems to be that C# exceptions that would
normally be caught by the C# code are instead causing SIGSEGV
faults and crashing the app.  Use WESAY_INIT_XULRUNNER=0
to disable initialization of XulRunner (still enabled by default).

Change-Id: I54dda3443e13ccd34b6ed336fe680c9e59d88cf8
version 1.5.15

We are still having crashes when XulRunner is initialized.
Use WESAY_INIT_XULRUNNER=0 to turn off XulRunner initialization.

Change-Id: I6990fc3a3262c4ec359137b563fdafc64ce670a7