* 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
* initial pass at indexing block header hashes by commitments
* check block height matches currently indexed block height
when getting block header for an output commit
* add some test coverage
* document get_block_header_by_output_commit
AntiochP