Add documentation for onepass.encryptKey()
Use web workers for PBKDF2 key derivation in browser

Avoid blocking the UI during password key derivation by moving
the work to a web worker.
Use same layout for scripts in development and published version of app

Put script bundles in webui/scripts/ instead of build/ so that
index.html and onepass_crypto can reference the same script paths
when doing local development and in the published version of the app.
Parallelize PBKDF2 processing

Calculate each of the two blocks of the output key in a separate
web worker.