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.
Merge pull request #202 from dcousens/ecneg

ECKey -D test and light cleanup
Merge pull request #203 from dcousens/hdwallettests

HDWallet tests and strict constructor
Base58check: no longer encodes version separately
EC/Key: use network object consistently
JSHint: add config
ec: use x1/x2, y1/y2 variables

No logic change, just using the variables that are defined instead of
repeating code.
JSHint: remove defined but never used
convert: use Buffers and add more tests
convert: use Buffer internally and remove unused functions
convert: remove reverseEndian