Move to BSD 3-clause license.

For some reason, Hackage has no intrinsic support for the BSD 2-clause license,
which I was previously using, I moved over to the BSD 3-clause license. I would
prefer the 2-clause license, but it isn't worth the extra effort.
Removed matrix module from the build file.

The matrices are not tropical yet (i.e. haven't written semiring instances for
them yet), so I'm excluding them from the build until I get around to doing
that. The matrices are sort of at the bottom of my to-do list at the moment.
Polynomials are next!
Added scalar exponentiation to the scalar code.

I was planning on using this time to add polynomials. However, I realized that
polynomials are just functions, and they don't really need their own module.
Instead,  I wrote things that you might use in a polynomial - namely exponents.