Merge branch 'feature/void_t' into 'master'

void_t, real_t, field_t in dune/common/typetraits.hh

Contains:
 - void_t (+voider)

 - template <class T>
   using field_t = typename FieldTraits<T>::field_type;

 - template &lt;class T&gt;
   using real_t = typename FieldTraits<T>::real_type;

Requires C++11.

See merge request !20