WIP - DTLS for RIOT

Here the obligatory pull requests for DTLS. Implemented functionality:

* basic handshake
* exchange encrypted data through a server and a client
* supported cipher suite AES CCM

Commit history and code needs refactorization. Not all commits are authored by me but rather as well by @l0rn (some rebasing went wrong...)
Huh, calloc is only missing for MSP?