SAMD CORE 1.6.3

* Added SPI.transfer16(..) method

* Bugfix: added missing Serial.begin(baud, config) method. Thanks @tuxedo0801

* Bootloader: 32Khz external oscillator is now used, improves USB clock. Thanks @aethaniel

* Bootloader: Clean up of makefiles and file organization. Thanks @aethaniel

* Added PWM capability to pins A1/A2 of Arduino Zero.

* variant.h/cpp: PWM capability is now determined by PIN_ATTR_PWM combined with

PIN_ATTR_TIMER or PIN_ATTR_TIMER_ALT for timer selection.

SAMD CORE 1.6.2 2015.11.03

* Fixed bug in delay calculations

* Fixed deadlock conditions in Wire. Thanks Erin Tomson

* Print not aborting on write() failure. Thanks @stickbreaker

* SPI can now be configured in variants. Thanks @aethaniel

* Implemented Wire.end

* Fixed external interrupt for pin 8 and 9

* Implemented Wire.setClock. Thanks @PaoloP74

* Wire: allow scanning bus via beginTransmission-endTransmission

* USB Device: big refactoring and bug fix

* USB Device: added PluggableUSB interface

* updated bossac to version 1.6.1-arduino

SAMD CORE 1.6.1 2015.07.21

* Added missing ATN pin definition

* Added missing bootloader key/value pair in boards.txt (this allows to burn bootloader with Sam-ICE too)

* Fixed min/max functions

* Fixed hooks for Cortex-M RTOS

* Fixed acknowledge of UART errors

* Enabled C11/C++11

* Fixed initialization in USB Device

* Updated bossac to version 1.6-arduino

SAMD CORE 1.6.0 2015.06.15

* First public release.