Added NetworkClass::RSSI

This function returns the WiFi signal strength in -dB [-127 to 0].
Error Values: 1 [CC3000 Problem]; 2 [Function Timeout/Bad Data]. It
reads through all entires in the table, continuing even after finding
the correct SSID; this prevents stale entries on the next call. There
is a one second timeout on the function for safety; this also forces
the function to re-run if the results table is corrupt. Note: The
CC3000 only updates the RSSI status every few minutes, so this function
doesn't need to be called constantly in the main loop.
Merge branch 'master' of https://github.com/timothybrown/core-firmware

Conflicts:
	src/spark_wiring_network.cpp