(bibtex-reference-head): Allow spaces and tabs between

(bibtex-reference-head): Allow spaces and tabs between
opening brace or paren and key.  Simplify regexp.
(bibtex-type-in-head, bibtex-key-in-head): Decrement.
(bibtex-mode): Use new function run-with-idle-timer instead of
auto-save-hook for periodically parsing keys.
(bibtex-mode-syntax-table, bibtex-autokey-name-change-strings,
bibtex-autokey-titleword-change-strings): Support non-escaped
double-quoted characters (as with german styles).
(bibtex-field-string-quoted): Disallow lines like
author = "Stefan Sch"of"
until BibTeX supports them.