Wrap enquire.js to allow its consumption as an AMD module.

We wanted to be able to use `enquire.js` internally in our all-AMD environment, so I wrote this little patch to conditionally wrap enquire as an AMD module using the [UMDjs pattern](https://github.com/umdjs/umd)

It currently uses `grunt-rigger` for in-place replacement of source files in the AMD version (https://github.com/buildjs/rigger / https://github.com/DamonOehlman/grunt-rigger) and outputs to `dist/amd/`. To be honest I only really used this to avoid touching the core source in order to make maintaining our fork easier. 

The UMD template output works both as a global and as an AMD module, so if you were willing to accept a version that updated the default to use this pattern let me know and I'll submit a pull request that does that instead.