Add new checks in `Plugin_Header_Fields_Check`

[ernilambar]

While going through the Plugin Uploader in Meta, I found that following issues are considered blockers:

• Empty Description
• Empty Version
• Invalid Version - (Regex: |[^\d\.]| )
• Plugin URI and Author URI are same

It would make sense to incorporate these in PCP also so that plugin author will be aware of the issues before uploading zip for the directory.