Add package.json draft.

Hopefully this at least partially fulfils the request in #22. The
"author"[1] and "license"[2] fields are missing, as I didn't know what
to fill in there. Most other aspects seem to be ok. The package name
"evaporate" instead of "EvaporateJS" was chosen, following npm's
standard naming conventions[3], but feel free to change that, of course.

[1] - https://www.npmjs.org/doc/json.html#people-fields-author-contributors
[2] - https://www.npmjs.org/doc/json.html#license
[3] - https://www.npmjs.org/doc/json.html#name