Add support for boost::hash

This patch adds boost::hash as a fallback option for Dune::hash
if neither std::hash nor std::tr1::hash can be found. We only
have to include the appropriate header and import the name into
the Dune namespace, as the built-in extension mechanism of boost::hash
will automatically pick up the global hash_value() functions.

To test whether the mechanism actually works, the patch also adds
boost::hash to the list of hash implementations tested in
bigunsignedinttest.

Kudos to Steffen Müthing, see FS#1192

[[Imported from SVN: r7067]]