New rule comparision operators, improve rules documentation

This branch adds the following new comparison operators:

* ``iequals`` - Case insensitive equality comparison (trigger value needs to be string).
* ``contains`` - String contains comparison.
* ``icontains`` - Case insensitive string contains comparison.
* ``ncontains`` - String doesn't contain comparison.
* ``incontains`` - Case insensitive string doesn't contain comparison.

On top of that, it also improves rules documentation section.

Note: Those operators act as utility functions. Same result can be achieved using ``matchregex`` comparison operator, but using ``matchregex`` results in a much verbose and harder to read and understand pattern (e.g. `^((?i)(?!@stackstorm).)*$` vs `@stackstorm`). On top of that, with not using regex we also avoid pathological regex / regex ddos problem.