[B+C] Add Unbreakable API. Adds BUKKIT-5675

##### The Issue:
Adds API to toggle the Unbreakable item property. If this tag is enabled the item won't lose durability on use.

##### Justification for this PR:
Unbreakable is a native minecraft functionality, this PR just adds the ability to toggle it through Bukkit's API

##### PR Breakdown:
* Add members to toggle Unbreakable to the base ItemMeta API
* Modify CraftItemMeta to implement API and account for Unbreakable in equals/hashcode
* Add serialization of the unbreakable property, defaulting to no unbreaking. (Default behaviour in MC, since Unbreakable is only available through spawning)
 
##### Testing Results and Materials:
The implementation was tested through a plugin using 3 commands:

* **/ubreak bow** spawns a bow with unbreaking enabled, doesn't lose durability when firing
* **/ubreak eqcheck** performs hashcode and equals checks on two ItemMeta's, with one being a clone with unbreaking enabled. It shows not equal for modified itemmeta, equality between clones was checked to verify it was still correct.
* **/ubreak serial** serializes an itemstack with unbreaking to a string and back to an itemstack, verifying the equality between the original and the serialized version.

[Test Plugin JAR](http://serverpeon.net/unbreakable-test-1.0-SNAPSHOT.jar) or [Test Plugin Sources](http://serverpeon.net/UnbreakableTest.zip)

##### Relevant PR(s): 
 
B-#### - https://github.com/Bukkit/Bukkit/pull/#### - Add Unbreakable API  
CB-#### - https://github.com/Bukkit/CraftBukkit/pull/#### - Implement Unbreakable API and serialization of the Unbreakable property
 
##### JIRA Ticket: 
BUKKIT-5675 - https://bukkit.atlassian.net/browse/BUKKIT-5675