…Register, portModeRegister

# WInterrupts
* change attachInterrupt define to return the interrupt mask
* callbacksInt, channelMap now uses an initialiser list instead of memset
* GPIOTE_IRQn priority dropped from 1 to 3 (same as Uart priority)
* LOW and HIGH modes added to attachInterrupt for compatibility, which are the same as FALLING and RISING respectively as GPIOTE does not support HIGH and LOW modes
* To free up clock cycles between GPIOTE interrupts GPIOTE_IRQHandler now searches for the first GPIOTE event and breaks to execute, rather than staying in the interrupt and executing each GPIOTE callback that has triggered an interrupt.

…ease 1.0.1

* merged SoftwareSerial.cpp into SoftwareSerial.h to allow ```_SS_MAX_RX_BUFF``` to be user-defined
* added ```_SS_TX_ONLY``` user define to allow this to be used as a transmit only library (no interrupts used)
* removed workarounds for legacy versions of Arduino that arent supported by this core

* Added separate LICENSE file
* Updated Copyright information
* Bugfix: replaced #ifdef / #ifndef _SS_TX_ONLY with #if _SS_TX_ONLY == 1/0
* Updated TransmitOnly.ino to demonstrate the use of _SS_MAX_RX_BUFF