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.
Agree the `Serializable` could be improved.  This changeset doesn't touch its state and so doesn't address its serializability. :smile: