Build: Split out End-to-End crypto library from extension

[coruus]

There's no pure-JS OpenPGP (or crypto) library of similar quality.

The crypto really merits its own project, especially as many Node.js people are using rather poorly written alternatives.

See YahooArchive/end-to-end#32 for the parallel issue.

[willscott]

I have periodically been updating an NPM package tracking the library

https://www.npmjs.com/package/e2e

https://groups.google.com/forum/#!searchin/e2e-discuss/npm/e2e-discuss/z4wo947DN3Q/kh3T5rdrBPYJ

[coruus]

@willscott: 👍

[koto]

This is an ongoing project. There is a build target that one can use without the extension code: ./do.sh build_library. More documentation, code examples and such are incoming.