Fixed a bunch of bugs, and some documentation issues

I fixed a number of issues to make the validator work against [Project Open Data](http://project-open-data.github.io/schema/).  Some of these are:

*  Made the "Required If Available" properties not required, unless you check a checkbox.
*  Created a checkbox to make the Extended attributes required
*  Updated the link to the schema definition on Project Open Data
*  Fixed several date formats to reflect [Project Open Data](http://project-open-data.github.io/schema/)
*  Fixed keyword to reflect  [Project Open Data](http://project-open-data.github.io/schema/)
*  Fixed required keyword to include all the required properties from Common Core in [Project Open Data](http://project-open-data.github.io/schema/)
*  Fixed the example JSON to use one of the examples from [Project Open Data Metadata Resources](http://project-open-data.github.io/metadata-resources/)