Extracted built-in class completion to phpcomplete#CompleteBuiltInClass()

This way it can be tested separately.
Extracted user class completion to phpcomplete#CompletePHP

Also fixed completion for real "static" methods and variables
when g:phpcomplete_relax_static_constraint is enabled.
Clean up tags file, it should not be part of the repo

Moved TAGS file to fixtures/GetClassName/ since this is the test it
actually uses it.
Created a more scoped tag file for the phpcomplete#GetClassContents tests.
Extracted class name completion for "new" or "extend" contexts to phpcomplete#CompleteClassName