[B+C] Add getResourcePack(). Adds BUKKIT-3615

##### The Issue: 
Most values from `server.properties` are exposed via `Bukkit` and `Server`, however the `resource-pack` value is not.
##### Justification:
This is needed to expose the `resource-pack` value of the `server.properties` file. It can be used for re-sending the resource-pack to the player, or just for info in general.
##### PR Breakdown:
Adds `getResourcePack()` to `Bukkit` and `Server`, and implemented in `CraftServer` like other `server.properties` values.
##### Testing:
Test plugin here: https://github.com/kenzierocks/TestPluginBukkit3615
It logs the value of `resource-pack` on startup.
##### Relevant PRs:
CB: https://github.com/Bukkit/CraftBukkit/pull/1355
##### Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-3615
There should either be a `<p/>` here or the two sentences combined into a more fluent format.