Better dialog to save attachments.

This commit replaces the call to `read-directory-name' by a call to
`read-file-name' when asking the user about the target location for
saving an attachment. This new dialog is better suited because:

1/ the user is currently saving a file, not a directory: he might want
   to complete over existing files (e.g., to replace an existing file
   with a new one just received).

2/ the current dialog was asking for a directory but then, if the user
   enters a directory, mu4e proposes him to "override" the directory
   with a file. This is not going to work obviously. In the new dialog,
   it is also possible to enter a directory name instead of a file name.
   In this case, a new file is created under the chosen directory name.
Update mu4e.texi for Fedora command

The currently pescribed installation for webkitgtk3-devel on Fedora is:

    sudo apt-get install webkitgtk3-devel

Instead, it should be:

    sudo yum install webkitgtk3-devel
Merge pull request #328 from robstewart57/patch-1

* update mu4e.texi for Fedora command
Merge pull request #326 from DamienCassou/dialog-to-save-attachments

* better dialog to save attachments.
* cosmetics
* minor update
* mu-store-read: cleanup function a bit

  (fruitlessly trying to track down
       https://code.google.com/p/mu0/issues/detail?id=80)
* minor change

  (try to workaround
      https://code.google.com/p/mu0/issues/detail?id=79,
   though I can't reproduce it at all)
* mu4e: add mu4e-compose-signature-auto-include and document it
* mu4e: check that message-signature is a string