Fix require with relative paths.

FIx require functionality to work correctly with relative paths
and make the implementation more robust.
Use relative URL when loading mini-module.js.

When serving these files from another location, loading mini-module.js
failed since src in script tag was using absolute URL. Change src to
relative to prevent this failure.
Re-factor tests to use mocha.

Modify tests to use mocha and cover a bit more test cases.
Remove server.js.

Running tests and serving files is done by mini-components top level
repository that serves basic components and allows integrated tests.