Added DETECT_CONNECTED_DEVICE environment variable to launcher

Some Ruby implementation like JRuby has trouble executing a process in
the Timeout::timeout(). JRuby won't throw the exception and hangs until
the process returns.

In Calabash launcher, it detects the connected devices by executing the
external process: udidetect. udidetect hangs when there are no devices
connected. Calabsh launcher executes udidetect in a Timeout and when it
timesout, calabash kills the udidetect process.

In Jruby, because of a JRuby bug, this causes the process to wait until
the launched process exits. TimeoutError is never fired.

This commit introduces an environment variable DETECT_CONNECTED_DEVICE
which can be used to turn on/off device detection. By default device
detection is on.