Improvements

Hello,

Don't merge this yet, there are other commits incoming.

In the meantime, while testing I had some issue trying complicated filters... Basically I was trying the following inside the dired-hacks folder:

* Select files olders than the 22/03/2014 `/ e (> (float-time mtime) (float-time (encode-time 0 0 0 22 3 2014)))` 
* Remove the .git directory `/ h`
* Negate the dotfiles filter `/ !`

So far so good. Now, trying to do the same with `*` instead of `/` and it'll fails, because the doc says:

    To remove marks that would otherwise be selected by a filter, use
    prefix argument (usually bound to C-u). To logically negate the
    meaning of the filter, you can call the function with a double
    prefix argument (usually C-u C-u)

1. How do I negate AND unmark? should we use `C-u C-u C-u`? that looks a bit weird.
2. How do you you negate the results of all filters? `/!` only negates the last filter.
3. `/ TAB` doens't seem to refresh the buffer, but doing `M-x dired-filter-transpose` does. Pressing `g` after doing `/ TAB` shows that it actually worked, no clue why it doesn't refresh.