An ebuild for deal.II [1]

This commit introduces an ebuild for the deal.II library.

[1] http://www.dealii.org/

 "deal.II is a C++ program library targeted at the computational solution of
  partial differential equations using adaptive finite elements. It uses
  state-of-the-art programming techniques to offer you a modern interface to the
  complex data structures and algorithms required."

Please note that p4est is not a mandatory build time dependency, so the two pull requests are independent of each other.