BufferWriter().toBuffer convenience method

It does the same thing as .concat(), but may be easier to remember, since the
rest of the library uses the ".toBuffer()" convention
add support for Dark Wallet-style addresses

These functions are prefixed DW which stands for Dark Wallet. The code for the
Dark Wallet address format can be found here:

https://github.com/darkwallet/darkwallet/blob/develop/js/util/stealth.js

Note that I deliberately support only the simplest possible format, which is
where there is only one payload pubkey and the prefix is blank. I should now go
back and replace my old toString, fromString, toBuffer, fromBuffer functions
with these Dark Wallet versions, since they are much more well-thought out than
mine.