Use browser field in package.json instead of process.verions tests

Replace process.versions tests by browserify's package.json browser field. 

I replaced all files in the lib/ directory by their matching file in node/ since they are no longer needed. I also got rid of Bignum altogether since it was just a dumb file for dispatching the correct module depending on environment. It's still exposed on Bitcore though.

All tests pass, including browser tests.

Fixes #329.