Build version of numeric suitable for amd

Adds a new build object, `numeric-$ver.amd.js`, which is a version of numeric suitable for [AMD](http://requirejs.org/docs/whyamd.html). This file is created by catting a simple prelude and postlude around the existing library.

Includes changes from #45 that are required to allow numeric's compiled functions to work even when the `numeric` symbol is not in the global scope.