Created LessThanOrEqualFilter and GreaterThanEqualFilter

I've copied and modified both LessThanFilter.php and GreaterThanFilter.php and created "Or Equal" versions of them.
This is mainly to allow their use in DataList filter() queries, see the example below.

Model::get()->filter("Field1:LessThanOrEqual", $value);

Currently, if you want to do <= filters, you can't do it with filter - this solves that problem.