UP-4330 MarketplaceEntry equals(), toString(), license header, and type JavaDoc.

 * Implements `equals()`, `hashCode()`, and `toString()` on `MarketplaceEntry` ( [UP-4330](https://issues.jasig.org/browse/UP-4330) ).
 * Adds Jasig license header.
 * Adds pro forma JavaDoc.
>This changeset doesn't touch its state and so doesn't address its serializability

Another thing to backlog perhaps? 

I am ok with direct commits (or pulls with no JIRA attached) for simple changes like this, but I also think there needs to be a way to remind ourselves of the task. 

Maybe time to move to Github issues? (Another JIRA to backlog this too? :))