Add a basic ringtone to utox.

Many users including myself want/need an audible ring tone to know when they are receiving call. (see issue#269) 

This pull request adds a ring tone that is synthesized from within utox(no external wav files are used or needed) and is played with the utox openAL.

The sound is played when a call is received. The sound stops playing after 10 seconds, or when the call is answered, which ever comes first. The sound rings for 2 seconds, pauses for 2 second and then repeats, much like a telephone ringer.

This pull request also adds a new  yes/no option to the bottom of the utox settings panel that allows the user to enable or disable the ringtone. It is disabled by default. Their selection is saved in the utox settings file and loaded when uTox starts.  

![ringtone](https://cloud.githubusercontent.com/assets/8443656/4690662/7a025404-56f3-11e4-8653-9fa42c0eca48.png)