New package for ECDH and ECDSA

I have added this http://kmackay.ca/micro-ecc/ to RIOT OS. This functionality is useful to negotiate an key over an unencrypted channel or to sign data.

I have tested the test procedure on an native system and on an nRF51822 (PCA10000) CPU. On native target sometimes signature tests are failing. It looks like native random function not serving data. When i start the same binary after some seconds all tests are fine.

It looks like the native random function is not ready for cryptography, because it uses the C pseudo random function. Is this right?

I can change micro-ecc package to use original micro-ecc random sources and use periph/random only on non native targets.