Reduced size of JSXTransformer.js by 10%.

Decided to add this small part from #1234 that uses native browser's `btoa` function for Base64 encoding and so allows Browserify not to include browser version of Node.js `buffer` module.

Comparison table:

             |     raw |       gz
-------------|---------|---------
**before**   | 421 641 |   84 774
**after**    | 384 700 |   76 649
**diff**     |  36 941 |    8 125
**diff (%)** |      9% |     -10%