Add iequal operator for performing case-insensitive equality check on strings.
Add new criteria comparision operators:

- contains
- icontains
- ncontains
- incontains
Add tests for new operators.
Use better argument names - v1 and v2 are ambigious and it's not clear which
argument is value and which one is pattern.
Use keyword arguments so it's more clear what is going on.
Improve rules criteria documentation and document all the available operator
types.
Update changelog.
Fix formatting.
Merge branch 'master' into new_comparison_operators

Conflicts:
	CHANGELOG.rst
Merge pull request #936 from StackStorm/new_comparison_operators

New rule comparision operators, improve rules documentation