[B] Add bulk permissions operations.  Adds BUKKIT-426

##### The Issue

Currently PermissionAttachments only allow the changing of one permission at a time, recalculating the Permissible's permission cache from scratch each time.  This can be non trivial when many permissions are registered and many permissions need to be altered.

##### Justification and breakdown

This PR adds three methods enabling bulk changing of permissions. ````addPermissions```` adds new permissions (or alters existing ones) from a map, ````setPermissionsTo```` explicitly sets the permissions of the attachment to the supplied map, and ````unsetPermissions```` removes permissions specified in a list.  As the behaviour of existing methods has not been touched (though two superfluous recalculations have been removed) this is not a breaking change.

##### Relevant PR:

This builds upon @Evenprime's PR #466 (though note that implementation is missing the ````this```` keyword and so never adds the permissions to the attachment) and also adds the ````setPermissionsTo()```` method.

##### Potential improvements

It is not possible to have both a <Permission> and a <String> version of these methods with the same name, as the method signatures would be identical (Object).  Like @Evenprime I felt that the String methods were more useful; if both String and Permission versions are desired, I would appreciate guidance on how to name both sets of methods.

##### JIRA Ticket:
BUKKIT-426 - https://bukkit.atlassian.net/browse/BUKKIT-426

##### Testing

Unit tests have been added in org.bukkit.PermissionsTest.java and all pass

Test plugin: https://github.com/frymaster/permissions-test

This plugin exercises the new methods and logs the resulting permissions.  Expected output is as per the comments in the file ( https://github.com/frymaster/permissions-test/blob/master/src/main/java/org/_127001/frymaster/permissionstest/PermissionsTest.java )

Compiled jar file: http://frymaster.127001.org/files/permissionstest-1.0.jar

Output: https://gist.github.com/frymaster/5723247 is as expected
There should be a gap between import groupings.