Wrapping the exported global in AMD

The AMD wrapper allows for dom.js to be `require()`ed into an AMD loader
context like RequireJS. This shim is the UMD web global variant found in
https://github.com/umdjs/umd

The original global `DOM` reference is preserved in both the AMD case
and normal global case.
Merge pull request #1 from Aintaer/amdify

Wrapping the exported global in AMD