Add tests to ensure numericality validator with greater_than and less_then options works (currently failing)

When using money-rails 0.9.0 in a project, the numericality validator with the greater_than option didn't appear to be doing any validation, so I checked out the money-rails project and examined the tests. The existing tests only check that numericality validation is rejecting non-numeric values, so I added tests to check that the greater_than and less_than options are working on monetized fields as described in the readme.

These tests are currently failing. It looks like the numericality validator on monetized fields is ignoring the greater_than or less_than options.

Ideally I wouldn't dump failing tests on you, but I don't have enough familiarity with money-rails to fix the issue myself.