Add basic support for hashing using Dune::hash

This patch introduces the functor Dune::hash in a similar manner to
other C++11 functionality by importing the definition from namespace std.

Apart from that, the patch also provides some basic functionality to simplify
making user-defined types hashable, in particular functions for combining hashes
and hashing iterator ranges as well as a mechanism for defining the struct
specializations required by std::hash and std::tr1::hash in the correct namespace.

The current implementation will first try to use std::hash and, if that fails,
attempt to fall back on std::tr1::hash. If that is not available either, hash
support is disabled as there is no homegrown fallback for now.
The extension mechanism will register the type with both std::hash and std::tr1::hash,
if available. This should make it possible to use unordered_{set,map} from both C++11
and TR1 without having to specify a custom hasher.

Kudos to Steffen Müthing, see FS#1192

[[Imported from SVN: r7065]]