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 
Yeh that's a good idea.  It's probably a good idea to avoid doing too much in one PR, however as this is easy to do and won't affect any other part of the code shall I add it in this?