Implement Keystore

[dariusc93]

Current implementation only allow passing in a single keypair, which would be useful for starting the node, however this implementation does not support a keystore that would allow the creation, storage and usage of generated keys. This would be helpful when it comes to implementing #1, or if a node which to select a specific keypair for starting their node, or possibly other usage (eg implementing signing or encryption for dag, though such specs are not finalized or complete)