Add failing tests for regex matching.

These are simply rudimentary tests for #1. We should add tests and
implement for any type which is convertible to strings, but I think that
for now simple string type regex matching will be useful.
Add basic implementation of regex matching.

This currently only works for string types. On a better implementation
it'd support making regex match assertions about any type convertible to
strings, but it's a trivial problem to solve at the client's side.

This, therefore, closes #1 for now and bumps version to 0.0.6.