* introduce grin_keychain, encapsulate derivation of secret_keys
* core compiles against keychain, tests don't run yet
* core tests are now passing against keychain
* wip - getting wallet working with keychain
* add util and keychain to travis test matrix
* basic test around key derivation
* wallet key fingerprint as hex string
* use lowercase hex in util::to_hex for consistency with various crypto libs, add some tests for util::to_hex and util::from_hex
* filter wallet info based on ext_key fingerprint
* cleanup format for fingerprints and output status