Extract comletion for variables to phpcomplete#CompleteVariable()
Remove "->" from the end of variables detected as class instances

The added "->" only helps you when you want to call a method on that
variable, but there are scenarios when you want to use just the variable,
like passing it to a function as parameter or using it in a string
concatenation / interpolation. Half of the times you will end up deleting it.
Added g:phpcomplete_complete_for_unknown_classes option

This option lets you disable the completion when in object context
but the code cant tell the class or find it (with tags).