[B+C] Add Location.getNearbyEntities. Adds BUKKIT-3868

## The Issue

There is currently an Entity.getNearbyEntities() method, but no way to perform a similar check on an arbitrary Location without an Entity.

## Justification for this PR

This PR adds the Location.getNearbyEntities method requested in BUKKIT-3868, as well as a World.getEntities(Location, x, y, z) method that backs it. These additional utility methods provide a way for developers to efficiently query for entities within a given area, without needing a source Entity.

## PR Breakdown

### Bukkit

Add Location.getNearbyEntities(double x, double y, double z) and World.getEntities(Location center, double x, double y, double z)

### CraftBukkit

CraftWorld implements getEntities similarly to CraftEntity, except it passes in a null source Entity (given current NMS code, this is safe, the entity parameter is only used with "==" to filter out the source entity) and it creates an AABB rather than using a modified Entity BB.

## Testing Results and Materials

A test plugin can be found here:

https://github.com/NathanWolf/Bukkit-Unit-Tests/tree/Fix-BUKKIT-3868
http://mine.elmakers.com/share/Test-Fix-BUKKIT-3868.jar

This plugin will search for entities at your target block (block under cursor) when you swing (left-click) a stick or blaze rod.

A stick will check within 8 blocks, a blaze rod within 64. It will print a list of entity counts by type in the area, and can be used to validate that the correct entities are found.

## Relevant PR(s)

CB-1384: https://github.com/Bukkit/CraftBukkit/pull/1384 - Associated CB PR

## JIRA Ticket
https://bukkit.atlassian.net/browse/BUKKIT-3868
Haha- again, this is all copy+paste .. I mean that's a poor excuse, but I tried to make these look like the existing getEntities methods.

That said, I'm changing all of mine to something like "A Collection of entities nearby", and leaving the old ones alone, e.g.:

```
     * @return A List of all Entities currently residing in this world that
     *     match the given class/interface
     */
    public <T extends Entity> Collection<T> getEntitiesByClass(Class<T> cls);
```