Make scrypt (C++ binding) optional

[sullivanpt]

Hi, please consider this PR to make the scrypt binary an optional dependency.

With this change an npm install will succeed even scrypt fails to build on the local host; in this case the pure JS version (scryptsy) will be used on both the browser and on node.

The "load specific version" will still fail if the demanded version is not present; however, the failure will occur at the time of the require rather than during npm install.

var scrypt = require('scrypt.js) // pure JavaScript (will ALWAYS succeed, might return pure JS on node)
var scrypt = require('scrypt.js/js') // pure JavaScript (will ALWAYS succeed)
var scrypt = require('scrypt.js/node') // C on Node (will throw an exception if scrypt not installed)

[davidmurdoch]

@axic Any chance this PR could be merged and published to npm?

[benjamincburns]

@axic to add some detail to @davidmurdoch's request - in ganache-cli we actually use webpack specifically to replace this dependency with a non-native one in order to simplify things for our users.

This decision was made a while back because we noticed at hackathons and training events many (windows) users were spending an exorbitant amount of time troubleshooting their native build environment.

This forces us to not only pack ganache-cli, but all of its dependencies, which makes the package much heavier than it needs to be, and complicates our testing process unnecessarily.

So that's really a lot of words to say that accepting this PR would alleviate a lot of pain for us.

[axic]

@sullivanpt @benjamincburns sorry for the delay, I haven't gotten the notifications for some reason :(

[axic]

Released 0.3.0.

[axic]

Also packaging tools supposed to use the "browser": "js.js", line from package.json, but this change gives a secondary options for those which ignore it.