Add idevicerun tool

Added new "idevicerun" command-line tool to run iOS apps via the debugserver service.

Starts app by name, e.g. `idevicerun com.foo.Bar`, prints output to stdout, kills on CTRL-C, and returns the app's exit code (if the app dies).

Some obvious overlap with idevicedebugserverproxy, 'though this new idevicerun adds the required gdb I/O for this common "run my app" case.

Please ignore 13be3c1 -- it's already merged in fb4b91d79.  I could rebase it away but it should be a no-op.