Merge pull request #4 from ARMmbed/psa-wrapper-apis-march-2

Interfaces: general framework for key types and algorithms; key import, export and destruction; multipart hash, MAC, cipher and AEAD operations; asymmetric sign/verify, key lifetimes and policies.

Implementation: key import, export and destruction (RSA, symmetric keys); multipart hash and MAC operations. Tested only with a few smoke tests.