Support STM32 Arduino core

Fix compiler warnings and pointer comparison error.

Pulled in memory leak fixes to address issue #2.

• Thanks @ROSW6341 !

Added support for and tested on SAMD21 boards, using the hardware serial port.

"00_Register_Operator" example was modified to more-easily set a different Serial Monitor and/or LTE Shield serial ports. This example still defaults to supporting AVR architectures, like the Arduino Uno, but can be modified to use the SAMD21 by:

• Comment out line beginning with SoftwareSerial lteSerial
• Set SerialMonitor to SerialUSB
• Set LTEShieldSerial to Serial1 (also set Serial-select switch on shield to "HARD")
• (Optional) Set the lte.begin baud rate to 115200 (hardware port can handle it!)

Initial public release of the SparkFun LTE Shield Arduino library.