Naive Rcpp Implementation

I wrote the _extremely_ naive algorithm in Rcpp. I used the [Eigen linear algebra library](http://eigen.tuxfamily.org/dox/). Figuring out its API was extremely tedious, but it was faster and more reliable than writing the multiplies by hand. I was trying to write the ordinary naive algorithm, but by accident, I ended up rewriting snappy in Eigen along the way.

In the end, snappy is only twice as fast as the ridiculously naive version. That says to me that nearly all of the speedup we're seeing is due to avoiding the R interpreter and avoiding the -- implicit copies there, which is sort of disheartening.  I suppose Lysy doesn't care how we got the result, so long as it's usable.

This now depends on [RcppEigen](http://cran.r-project.org/web/packages/RcppEigen/index.html), so run install.packages("RcppEigen") first ((when we get to packing this up we'll have to mark that as a dependency). Learning that was.. interesting. I think there's some scattered notes about what I learned along the way in these commits.

Algorithms are now listed in `test.constants.R`. When you write new ones, add them there.

I **also** discovered in kousu@2eafaeed57ed9706aadb25d8bca5e5d0939867c3 that `snappy1` was doing the factorization inside of each loop (the opposite of the bug you have with `Rcpp2`!). It is now on par with the other two, with a factor of 2 speed up.