Add DOFMap output class

As described in #4155 this output module dumps the degrees-of-freedom map in JSON format to a file and/or the screen. JSON is a widely supported text based serialization format. 

Besides the raw DOF data I'm also outputting active kernels for every subdomain (names and type names!). 
The main reason was me not knowing about ```libMesh::demangle()```. Not even ```MaterialProperty<T>::type()``` in MOOSE uses that. But yeah, that is of course a better option!