Improve context detection

Don't remove the last "word" when there were no base found. This helps in
situations where you are after a keyword like "extends" and trigger the
plugin.
Fix regression introduced in 8b376bf7d5

The a:base is always empty when we are in "findstart" mode, so the base
was never chopped down and prevented completion to recognize $this->some_
contexts.
Added misc/builtin_manual.vim for non-autogenerated builtin info

This new file can hold the hand-made additions supplementing the
auto generated hashes. For starters it holds the XMLWriter class's info
(it doesn't have the usual class info page in the manual).
Fix copy-paste generated error in interface support

Special thanks for @naglik who found and fixed this in his fork.
(in commit naglik/phpcomplete.vim@ccb8cabb8f)