bufferutils: return opcode for use
ecdsa: serializeSig should return a Buffer
ECKey: adds test to ensure throws on negative D
ECKey: use signum over compareTo
base58: use signum over compareTo
ECKey: ensure throw is from ECKey
base58check: buffers only, remove explicit 0

Undefined is treated as zero if it ever gets to this point anyway, so no
value is added by having this default param here.

A test fixture to verify this behaviour is added.
Address: Expected T, got PARAM for Buffer assertion
base58: cleanup variable names

The encode/decode functions are also now similar syntactically.