Add FM volume and signal quality, refactor to support flash strings.

Added new functions for controlling FM volume and querying FM station signal quality.  These are just simple wrappers around the lower level SIM800 FMVOLUME and FMSIGNAL functions.

Also added getReply and sendCheckReply overloads that support sending and verifying flash strings.  Refactored the FM radio functions to use flash strings as a start for reducing some of the RAM usage.

Updated example sketch with FM volume and signal quality commands (m, M, and q) and tested all the FM radio functions to verify they work as expected.