Add counters and pagestyle to some class styles.

* style/article.el ("article"): Add counters and pagestyles.

* style/beamer.el ("beamer"): Ditto.

* style/book.el ("book"): Ditto.

* style/letter.el (LaTeX-letter-class-options): New variable.
("letter"): Add pagestyles.

* style/report.el ("report"): Add counters and pagestyles.

* style/slides.el (LaTeX-slides-class-options): New variable.
("slides"): Add counters and pagestyles.
Some improvements to amsthm style file.

* style/amsthm.el (LaTeX-amsthm-package-options): New variable.
("amsthm"): Remove `newtheorem' macro, already defined in
`latex.el'.  Use `TeX-arg-define-environment' in `newtheorem*'
macro.  Add `qedhere', `swapnumbers', and `newtheoremstyle'
macros.  Add `newtheorem*' regexp to match new environments.  Add
fontification.