Enhance expectJSONLength to allow <, <=, >, >=

Previously only exact length matches were allowed, now allow
.expectJSONLength('properties', '<=10')
Enhance expectJSONLength to allow <, <=, >, >=

New test coverage for this new functionality of <, <=, >, >=. You can run the current test suite by running npm test.
Merge pull request #20 from danactive/patch-1

Enhance expectJSONLength to allow <, <=, >, >=