Added ability to play a sound upon receiving new fax

Audio files must be placed in includes/audio/ and can be mp3, ogg or wav. Didn't test yet browser's capabilities, just tested a mp3 file with Chrome 27.

There's no proper handling into the database, files are read upon config from filesystem and stored into user profile data.
Some minimum checks for avoiding read of other filesystem files is implemented.

The alert () prevents the execution of the file so I had to exclude it in case of audio playing.

Maybe a config option should be added for this.