Watchdog and select() support

1. If USE_WATCHDOG is defined, resets the watchdog timer at appropriate spots.  (The watchdog timer is a feature which automatically resets the chip if the timer isn't reset after a period of time, allowing the micro controller to recover from occasional hangs). Generally, the watchdog timer is reset immediately prior to communications with the CC3000, but not during inner loops.  Thus, if the CC3000 hangs, the watchdog is capable of resetting the micro controller.  (And the micro controller will then presumably power cycle the CC3000)

2. Implemented the select() socket function.  This is used by, among other things, the Adafruit CC3000 library.