use a different way to re-build asmcrypto.js (fixes travis build)

- when npm 1.4.9 installs asmcrypto, it already includes the asmcrypto.js file upon install which means our rebuild target was never called
- also add some variables to set different NPM and NODE
- make npm install commands more precise to avoid overwriting other deps
- add a prepublish script for publishing a top-level jodid25519.js to npm repos

The last one is meant to be easier to use than referencing build/jodid25519-shared.js directly.