Add namespace awareness in classnames when searching for location

Made the class location lookup take namespaces into account when the tags
have a namespace field
Made phpcomplete#GetClassContents aware of namespaces when trying to get
the extended class's location
Made class name detection aware of namespaces in class names
Related to #4
Add namespace awareness for classname completion in "new" and "extends" contexts.

Added filtering on current or typed in namespaces when tag files
have namespace field.
Related to #4.