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
Merge branch 'feature' of https://github.com/mybb/mybb into feature
Fixing code style.
Merge pull request #171 from MyBBStuff/feature

Object/Class method support for plugins