Facts and Attributes should be non-negative instead of positive

Zero should be an allowed value for these integers.
Fact data typing changes

Facts that are typed as positive integers are now classified as non-zero to allow 0.
Result of bad fact type will now result in the fact having no value.
The create or update will finish without the value for the fact.
Merge pull request #157 from candlepin/wottop/positive-facts-can-be-zero

Wottop/positive facts can be zero