Review "main" and "browser" entries on package.json

[zenorocha]

Both browserify and webpack relies on mainand browser fields and we want to improve the user experience when requiring this module.

Related to #15 #27

[zenorocha]

This includes moving browserify and babelify out from dependencies to devDependencies https://github.com/zenorocha/clipboard.js/blob/master/package.json#L25-L26

[vccortez]

So, as far as I know, the browser field is only used by module bundlers in its simplest form (a string value pointing to the alternative main file).

If for some reason module bundler users prefer to work with unbundled files, then as pointed out by @daviferreira, we could, for example, add a browser folder with babel compiled sources (not yet bundled), and set the browser field to point to browser/clipboard.js.

I submitted a pull request in #106 to address the browser field issue.