Closure support for plugins

Plugins can add hooks in the form of Closures to speed up develoment of
smaller plugins.
Removing spacing
Adding support for class methods as well as closures.

Class methods can be passed as an array. You can use either instance or static methods. To use an instance method, consider the following:

    class TestClass
    	{
    		function test($page)
    		{
    			$page = str_replace("<div id=\"content\">", "<div id=\"content\"><p>Hello World!<br />This is a sample MyBB Plugin (which can be disabled!) that displays this message on all pages.</p>", $page);
    			return $page;
    		}
    	}

    	$obj = new TestClass();

    	$plugins->add_hook("pre_output_page", array($obj, 'test'));

 To use a static method, simply pass the class name as the first array parameter and the method name as the second.
Removing stray echo
Adding remove_hook functionality.
Merge branch 'feature' of https://github.com/mybb/mybb into feature
Fixing code style of object support for plugins.

Signed-off-by: Euan Torano <euantorano@gmail.com>
Merge branch 'feature' of https://github.com/mybb/mybb into feature
Delete user improvement

Improves the method for use deletion.
Fixes #216

Fixes #216: Instant activation not working as expected
Addressing #425 for Feature again

. . . where deleted users' reports were not labeled
correctly.
Addressing #463

. . . where default report reasons were not sent correctly when logging
user reports.
Amending 952a905 for #425 for Feature

. . . where deleted users reports were unlabelled in 'View All Reports'.
Addressing #290 for Feature

. . . where user group setting for ability to add a signature was not
working correctly.
Fixes #343 missing "Update Attachment" when reaching "maxattachments"-setting
WIP #431
WIP #428
WIP #428 Selectable areas for group/forum settings

Cleans settings.php
WIP #428 Selectable areas for group/forum settings

I forgot the obvious ones :P
WIP #428 Selectable areas for group/forum settings.

Updates the upgrade script for post merge settings.