Renaming limit names
Moving clause-sort to cnf.cpp, removing dependency on clauseallocator.h

This makes files less dependent on change to each other.
Refactoring clause cleaner to use sub-functions
Adding a new limit for empty resolvent elimination
Starting empty resolvent elimination at a random place
Renaming a function to better reflect what it's doing
Print status line more often

Fixes #113
Removing useless comments