Added getMailboxes & + working utf7 functions

**getMailboxes(true)** permit to have the list of folders like this:

![mailboxes](https://cloud.githubusercontent.com/assets/1408020/2558329/aaacfcc8-b741-11e3-9312-b6e7db37adf1.png)


**imap_utf7_encode** and **imap_utf7_decode** not working correctly with "éàèêëç...", I found the solution here: 
[FR] http://www.developpez.net/forums/d1179766/php/langage/php-imap-probleme-utf7/

I've made a non backward compatible **__construct** function but if you have a better proposal I can change it.

The code has been tested.