Rebuilding

Add methods to Asset classes to rebuild the Asset data from the
payload. Because it is inevitable that I will want to add new
properties to an Asset, and should not need to pull everything from
sources again.

For example, also added the permalink and target_url properties:

permalink: the link to this asset within the original system
target_url: for assets that are links to other assets, such as Reddit
posts or Pinboard pins, record the target of the link