Crypto: encrypt/decrypt, move cryptopp to implementation, tests.

Note: ECDH and ECDHE key exchange are still under development. I would like these schemes to be reviewed however encrypt/decrypt is primary focus; ECDH-related classes and tests will be removed along with ECKeyPair prior to merge.
Please replace by
```
assert(_p.size + 1 == prefixedKey.size()); // even better: compile-time assert, if possible
memcpy(_p.data(), &prefixedKey[1], prefixedKey.size() - 1);
```
And I also don't like the memcpy. It would be great if we could do
```
_p = bytesRef(prefixedKey.begin() + 1, prefixedKey.end());
```