UP-4330 MarketplaceEntry nicities

 * Implements `equals()`, `hashCode()`, and `toString()` on `MarketplaceEntry` ( [UP-4330](https://issues.jasig.org/browse/UP-4330) ).
 * Adds Jasig license header.
 * Adds pro forma JavaDoc.
Strategically, yes.  Tactically, not a change to make in this changeset.  Switching that header should be done throughout, presumably, and should involve bumping the license plugin configuration. Backlogged as [UP-4361](https://issues.jasig.org/browse/UP-4361).