Change signature of method startSSIDscan

The return value of method startSSIDscan has been used for two purposes; the number of networks (when executed successfully) and status reporting (return false). To better distinguish between (real) results and status information, the method's return type has been changed to bool. The original return value (number of networks) is now a reference parameter.

The type of the reference parameter has been changed to uint32_t according to field num_networks of ResultStruct_t.
Merge branch 'signature-startSSIDscan' of https://github.com/philippthun/Adafruit_CC3000_Library into philippthun-signature-startSSIDscan
Merge branch 'philippthun-signature-startSSIDscan'