Fix `Matrix2::from_angle()` invocation and add a test for the feature.

I originally noticed this oddity when writing the documentation examples for Basis2, but was not sure why it was occurring. The current `Matrix2::from_angle()` will rotate vectors clockwise, as opposed to counterclockwise.

If it is an error, it's probably caused by someone forgetting that the `new()` method takes entries column-wise, not row-wise. But this might be intentional; there is a mention in [Wikipedia's definition](http://en.wikipedia.org/wiki/Rotation_matrix) of a rotation matrix in computer graphics preferring clockwise rotation.

I have also added a test function to the matrix test suite and fixed the example for Basis2 to account for this change.