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.
...and frankly, if you care, importing a static method and calling it down below is two lines. Calling it outright is one! I don't really see much value in not using static imports in a wildcard manner for methods but I could be convinced otherwise :)