Limit use of non-primitive types throughout the API

[dcousens]

As a result of the discussion in #359, the aim will be to limit the use of non primitives in the API.

Buffer is considered a primitive in this case, and is therefore acceptable.

[dcousens]

Wherever the API changes, warnings will need to be added to 1.5.x.

[dcousens]

Best I can tell, the only non-primitive constructor that we have is in function ECPair (d, Q, options).

[dcousens]

I didn't realise before, but Script is a major component which could be simplified down to a Buffer for interop.

With modules like https://github.com/bitcoinjs/bip69 coming up, the ability to reduce this part of the API would be asimilar to the changes made to Address.

And therefore highly beneficial.