Added package.json so that zxcvbn will work with npm

Hi, I wanted to use this with npm, and searching around I found that the current library in npm is broken and perhaps not maintained as actively as would be desired...

In any case, I thought it might be better if it were all in one place?

With that in mind, I've added a few files so that this will work with npm:
package.json
.npmignore - make sure the final package is trim, taught & terrific
index.js - optional, but makes things neater when requiring