Custom Fatal() and Fatalf() functions
A primitive implementation of the 'exec' op, which simply executes the command with no isolation
New op: 'echo' for debugging the engine
Send engine commands using a redis-style protocol
Small improvements to the 'build' operation: skip empty lines and comments, and use the new engine protocol
Simplify the code attaching command stdout/stderr to engine's stdout/stderr
FIx a bug in command chaining
Don't enable debugging by default
The DIE operation tells the engine to shutdown gracefully