set up npm prepublish
cleanup on npm postpublish
install grunt before travis calls npm install

apparently npm prepublish is called on npm install
package: add "browserify.transform" section

So that `quill` can be browserified from the command-line
after a simple `npm install quilljs` command.

See: http://stackoverflow.com/a/16113666/376773
package: add the "main" property

This still doesn't make browserify compile without
--extension=".coffee", but at least it gives a less cryptic
error message than "quilljs" can not be found.
package: add "browser" field, remove redundant "main" field