Just link to RcppEigen

* but we get this:

Warning message:
The following packages are referenced using Rcpp::depends attributes however are not listed in the Depends, Imports orLinkingTo fields of the package DESCRIPTION file: RcppEigen

which is obviously spurious (since it is listed in LinkingTo) and presumably an Rcpp bug,
but better than:

* checking package dependencies ... NOTE
Package in Depends/Imports which should probably only be in LinkingTo: 'RcppEigen'

which will offend CRAN
Bump version to 0.3
Merge branch 'fix/linking-to'

* fix/linking-to:
  Bump version to 0.3
  Just link to RcppEigen