Generated definitions are not usable in a browser

[tamird]

Broken in 0c90364. That commit introduced hardcoded references to Buffer which is not defined in browsers, breaking Proto2TypeScript for non-Node environments.

From #13 (comment):

But we should definitely fix that. The flag to enable the node-only definitions is a good idea. The tests could also be improved, they were develop when I was the single user of the project, and they are far from perfect.

@kalbasit @yellowiscool