add checkers: less_than, less_than_or_equal, greater_than, greater_than_or_equal
add document and examples to less_than, less_than_or_equal,
greater_than and greater_than_or_equal
Merge pull request #32 from yfractal/checkers

Add checkers: less_than, less_than_or_equal, greater_than, greater_than_or_equal