Add logic for using the 'gated' feature.

Adds `detect.js` file for `classList` and adds logic for pre-building gated (minified and non-minified) Javascript polyfills.  This also verifies the validity (in terms of Javascript syntax of the `detect.js` file as a conditional expression) at process start up.

Closes #5 
I'm not fan of generating property names, and you lose the ability to fall back nicely if the property doesn't exist in cases where `detect.js` files are not available.