Remove weak/node-gyp dependency

[swim]

When running Phantom on a Windows box it defaults to requiring node-gyp. However, this is not required; please see https://github.com/amir20/phantomjs-node/tree/v1#use-it-in-windows. This should also address an old issue #2.

[coveralls]

Coverage remained the same at 99.441% when pulling 093a911 on swim:remove-weak-dep into 4ae680b on ruipgil:master.

[ruipgil]

You can pass an object with those functions, when you're instantiating a dynamic scraper. I feel that this is unnecessary to be baked in. Maybe a note in the readme.