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.
static importing a single method is somewhat odd. Why not the whole class? or Not import at all?