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
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).
Make tagfile usage explicit in GetClassName testcase
Extracted "complete for unknown class" path to phpcomplete#CompleteUnknownClass

Redid the variable extraction.
Fixed scratch window's content for matched g:built_in_object_functions