Added buffer and async support

This PR adds support for `Buffer` and async callbacks.

- Inputs no longer constrained to 16-bit alignment
- Buffers returned by `crypto` methods can now be used without conversion
- When called asynchronously no non-critical failure errors are thrown
- Unused `four_letters.js` machinery has been removed
- Tests added for buffer, async, and the pgp-word-list dep
- README updated to reflect API change
- *Breaking change:* probably not easily ported to a browser now that Buffer support is caked in.