Decide how to proceed regarding DER keys format

[yago-123]

Right now we make use of DER encoded []byte private and public keys for wallets. The problem of this is that in some cases we must get rid of this encoding because we have to treat the keys with a raw format, example: handling the key derivation in hierachical deterministic wallets (which turns into something a bit ugly).

DER format have the advantage that provides additional metadata to the keys. This means that for example we can know which type of curve is a key with ECDSA algorithm. This is positive and negative thing. One bad thing is that we still use []byte format, which in some cases can create confusions with which type are we working with (DER encoded or raw).

Think about this and address somehow, a new type for DER formated keys could be created or maybe DER format must go away if the value added is not enough.