Added option to keep empty AST sub-trees (default to True) and a JSON parser example that needs this feature.
Added permutation operator. See comments in perm_rule method in plyplus.py and permutations.py example.
Merge pull request #6 from ray-harris-net/master

Added keep_empty_trees option to Grammar() class and permutation operator.

This commit break backwards compatibility, but this is the correct behavior, so I'm keeping it and considering it as a bug-fix.