[isf-wifidog] Wifidog validation email bug fix

[Proulx François]

I just fixed a major bug that has been around for many months. Our  
validation email (inside Ile Sans Fil) were not fully RFC822  
compliant, because we have non ASCII compatible characters in our  
UTF-8 subject lines... Some email servers would even totally ignore  
the email and discard them !!! I also realized that php  
mb_encode_mimeheader function do not return fully compliant data so I  
had to write a little bit of code... I created a new Mail class to  
keep it nice and tight. The class is stateless and 100% indenpendent,  
so it is reusable in any project.

This was a major issue for Ile Sans Fil since we might have lost a  
few users who never receive the confirmation email ! It's been tested  
on some of the most reluctant servers... We are now running it in  
production.

Have fun