Add linear regression via LAPACK

 - taken from R/qtl (src/lapackutil.c)
LICENSE -> License.md
Add License.md to .Rbuildignore; put copy in inst/
Add microbenchmark (within tests)
Small changes to pass R CMD check

 - get importFrom RcppEigen in NAMESPACE file
 - Seems we need a Makevars file to use LAPACK/BLAS libs
Don't need graphics in imports
Add ToDo list