HDNode: remove unnecessary slice operation
HDNode: shorten comment for Q validation
HDNode: clearer branch comments
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