* rename pubkey -> key_id, these are key identifiers, not public keys
* reduce identifier seize to 10 bytes, get rid of fingerprints (identifiers are now small enough to use as-is)
* IDENTIFIER_SIZE const
* add FeeDispute error for when sender and recipient disagre on fee calculation (should never happen)
* does not need to be mut
* cleaned up some warnings
* store wallet output data in hashmap
* cleanup up commented out code
* pass pubkey/identifier and not derivation around to miner etc.
* fix failing tests
* 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
