Add a good rotation example to Basis2.

This ads an example (the first?) on a useful feature of cgmath, and
also a Make target for running just the examples in the docs. This
may come in useful if more examples are added.

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Added `doctest` to the Travis config.

This requires removing the `make clean` step, but I don't think this
will have any side effects.

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Merge pull request #103 from atheriel/rot_ex

Add a rotation example to the documentation.