Added to_euler.

This adds a function to convert a Quaternion to Euler angles. It has been tested against cgmath itself, meaning to_euler -> from_euler should be a matching loop (with numeric losses of course).

I updated the from_euler to match the source document. I found the previous one did not work the way I expected it to, and was lacking any sort of tests. This is probably a breaking change for anyone who is using that function.