Enable ForwardSubstitution for Literals
Add missing keyword in omp statement
Rework openmp visiting forget nodes

* Many nodes were not visited but they are now
* Accept others nodes than ast.Name in omp directives
* Fix bad omp pragma
Fix forwarding for empty statement

* in case of : if foo(): i = 2; print i
  `i` will be forwarded and the if statement will contain nothing. It won't be a valid ast.
Merge pull request #226 from pbrunet/fix_forwarding

Fix forwading for empty statement