[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:
See Issue.
##### 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/TestPluginBukkit3561
It logs the value of `resource-pack` on startup.
##### Relevant PRs:
##### Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-3615