Add a dedicated Vendor Asset class

[JPry]

This adds the VendorAsset class for use with vendor scripts. The specific uses case is when handling a 3rd-party asset and registering a CDN URL for the asset instead of a local URL.

Notable changes:

1. Creates the VendorAsset class as an extension of the Asset class.
2. Adds a wpunit test suite to test various aspects of VendorAsset.
3. Makes some minor tweaks to docblocks in the main Asset class.

[dpanta94]

Other than the minor syntax comment - i like the work.

Going to ask @borkweb to take a look as well when he can before we merge though!

[dpanta94]

Just though that we would need to change the static method

Asset::add - to add a new static and not a new self!