Use std shared ptr

with this, we use std::shared_ptr instead of boost::shared_ptr. boost is not necessary for this anymore since we bumped the minimum GCC version to 4.4. While at it, I took the liberty to add a few config.h macros which should make the DUNE_DEPRECATED* and DUNE_UNUSED macros work correctly.

(I have corresponding patches for the other modules as well, but in order not to spam people too much, I will hold off with the PRs until we agreed on this one.)