cpu/board: Initial import of samr21-xpro

This PR provides basic support for the samr21-xpro board including

* UART I/O
* hwtimer
* GPIO/EXIT

This PR is meant to provide  a basic implementation for this board int RIOT's main repository to speed up distributed development and make the progress more visible. Development is coordinated via #1646 and documentation can be found in the [wiki](https://github.com/RIOT-OS/RIOT/wiki/Board:-Samr21-xpro)

**Caution:** Code will most certainly contain bugs.

This code also contains contributions of @Troels51.
I would actually move the call of `clk_init()` into `cpu_init()`...