General UMD support

135630e broke [UMD pattern](https://github.com/umdjs/umd), mentioned in https://github.com/Jxck/assert/issues/15#issuecomment-64975489 by @twada.
To resolve it, this pul-req introduces general UMD support to `assert.js` and `test-assert.js`.
use `data-main` attribute to bootstrap