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

Also [this line](https://github.com/Jxck/assert/blob/f25d9b80eac2cde3080613f4ef18a0a507dd4882/assert.js#L3) looks a bit unnerving:
```
// THIS IS NOT TESTED NOR LIKELY TO WORK OUTSIDE V8!
```
Spending time wondering if a problem was with the code, the test or the test library doesn't sound very lucrative.