Added free functions:

  dof_to_vertex_map
  vertex_to_dof_map

and deprecation warnings for the previous member functions in DofMap.
Small fixes in docstring
Merge branch 'master' into fix-issue-111
Add documentation of free functions: vertex_to_dof_map, dof_to_vertex_map
Start work on making compiled subdomain more robust
Merge branch 'master' into fix-issue-32
Change naming of compile_subdomains to CompiledSubDomain.
  -- Make CompiledSubDomain more robust, by enforcing setting
     default arguments (fixing issue 32)
Merge branch 'fix-issue-32'
Fix wrong type of returned {dof_to_vertex,vertex_to_dof}_map.
Merge branch 'fix-issue-111'