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. I am now fairly sure it is an error, probably caused by someone forgetting that the `new()` method takes entries column-wise, not row-wise.

See [Wikipedia's definition](http://en.wikipedia.org/wiki/Rotation_matrix) of a rotation matrix.

I have also added a test function to the matrix test suite and fixed the example for Basis2.