LDB operation

Design decisions for the first operation:
- numeric arguments are hex strings cast to long.js values as needed
- operations are prototype methods on an mmix machine
- each machine has its own memory and registers
- mmix.machines is a sparse map from hex string octabyte keys to hex
  string octabyte values; keys are padded to a full 16 hex characters
  to prevent dupe keys  such as "01" and "001".
- mmix.registers is a map from Registers enum type keys to hex string
  octabyte values
- mocha bdd table-driven tests
Merge remote-tracking branch 'origin/master'