Added completion for "use ..." namespace + classname contexts

Implements baseline completion for namespaces and classes.
Adds the let g:phpcomplete_min_num_of_chars_for_namespace_completion
option with default = 1, since taglist() is just too slow for a pattern
like '^'.
Related to issue #4, requires patched ctags to work properly.
See: https://github.com/shawncplus/phpcomplete.vim/wiki/Patched-ctags.