[B+C] Add getDamage method to Boat. Fixes BUKKIT-5325

#### The Issue:

The Boat entity is missing the getDamage method found on the Minecart entity.

#### Justification for this PR:

The `getDamage` method should be exposed through Bukkit, to allow plugin authors to get the damage on Boats, just like Minecarts.

#### Testing Results and Materials:

I've created a small plugin called BoatTester, which you can download [here](https://www.dropbox.com/s/yuh8k1so03b1995/BoatTester.jar). Load the plugin, then repeatedly left-lick on a boat in Survival mode. Messages will appear in the chat, informing you of the current damage. You can view the source code [here](https://github.com/Aaron1011/BoatTester).

#### PR Breakdown:

This PR adds the methods `getDamage` method  to the Boat entity.

#### Relevant PR:

CB-1327 - https://github.com/Bukkit/CraftBukkit/pull/1327

#### JIRA Ticket:

BUKKIT-5325 - https://bukkit.atlassian.net/browse/BUKKIT-5325


This could be reworded slightly to simply increase readability and understanding. An example would be `Gets the damage this boat has taken`.