simplify dependencies for browser tests

Currently, we include expect.js which we then wrap to conform to the API of Node's assert module. It's much simpler to add [Jxck/assert][1] as a dependency.

There's no need to have __mocha.css__ and __mocha.js__ in the repository, either. :)

Having removed these files, the only file remaining in __lib/test__ was __require-shim.js__, so I embedded it in __index.html__ once again. So long, __lib/test__!


[1]: https://github.com/Jxck/assert