Switched from browserify to pure-cjs builder

Switched to [recast](https://github.com/benjamn/recast)-based own [pure-cjs](https://github.com/RReverser/pure-cjs) CommonJS builder which allows to save ~10% of minified+gzipped library size.

Current comparison table is following (used output of `grunt-compare-size`):

orig   | orig.gz| diff.raw | diff.gz | filename                 | diff.raw % | diff.gz %
-------|--------|----------|---------|--------------------------|------------|-----------|
391132 | 77972  | 12212    | -10818  | JSXTransformer.js        | 3%         | -14%
538934 | 110908 | -5706    | -2861   | react-with-addons.js     | -1%        | -3%
111603 | 31270  | -15423   | -3162   | react-with-addons.min.js | -14%       | -10%
504173 | 103543 | -2489    | -2188   | react.js                 | 0%         | -2%
104632 | 29316  | -14674   | -2908   | react.min.js             | -14%       | -10%