attribute-value completion for haml, some refactoring

Changed
- ac-source-haml-attribute-value
+ ac-source--haml-attribute-value

In ac-html.el moved function named with "--" to begin
ac-source depended func located at end of ac-html.el file

Sad, but in haml mode I cant complete values in quoted strings.

So readme not updated for now.

However ac-source-haml-attribute-value work,
you can check in other modes using `ac-source-haml-attribute-value` source:

```lisp
(setq-default ac-sources
              '(
                            ac-source-haml-attribute-value
                            ac-source-haml-tag
                            ac-source-haml-attribute
                ))

;; fire as fast as can
(setq ac-source-haml-attribute-value
      (append
       '((requires . 0)
         ) ac-source-haml-attribute-value))

```
open your haml file, change mode to other (`text-mode`, for example)

type

```haml
%input(  :style="fon " :foo-bar :zzzz)
--complete here ok--^
```
but in `haml-mode` you can get value candidates only in such case:

```haml
%input(  :style="
--candidates-----^
```

but not

```haml
%input(  :style="" :foo-bar :zzzz)
--FAIL----------^
```