Fixed bugs in 2.0 schema.  Added new example file.

* **Fixed** Issue #130: the spec defines the "Headers Object" as an object, but the schema had it defined as an array
* **Fixed**: the "petstore.json" and "petstore.yaml" example files to use the "Headers Object" instead of an array
* **Fixed**: several properties of the "Parameter Object" that are defined in the spec were not defined in the schema
* **Added**: a new example file "product-catalog.yaml" that unit-tests all of the above fixes. It also demonstrates and tests many other features of the Swagger spec that aren't used in the other example files.
* **Fixed**: the Mocha unit tests were failing due to a known bug in the TV4 validator (https://github.com/geraintluff/tv4/issues/74)
* **Refactored**: the unit tests have been refactored to use js-yaml to parse **all** files (JSON _and_ YAML), since all JSON files are also valid YAML files.