Convert Buffers, not Byte Arrays

[dcousens]

This pull request changes the underlying types used by convert to use Buffer instead of byte arrays.
This pulls it into line with the rest of the code base, and removes some pointless conversions.

It also removes convert.reverseEndian, and adds more tests (and respective edge cases) to the convert tests.
Lastly, it fixes 2 incorrect (undefined) test descriptions in Address and HDNode.

[coveralls]

Coverage decreased (-0.04%) when pulling 943621f on dcousens:convclean into 63e6cf9 on bitcoinjs:master.