[RFC] Features/cipher operation modes

We try to implement DTLS as part of a software project at the Freie Universität Berlin. Therefor we need some block cipher operation modes which are not yet implemented in RIOT (there are only legacy headers files with no implementation at all). These commits restructure the current crypto module (these changes were made before Rene grouped everything into crypto) and add support for ECB, CBC, CTR and CCM (RFC 3610) modes. As this is marked as RFC we would love to hear your opinion about that.

Regards
Nico