added dtmfio module

# DTMFIO Module

## Description

Writes received dtmf button presses to a FIFO located at /tmp/dtmf.out.

Also, will write an 'E' when a call is established and an 'F' when the call is finished.

## To Do

+   Proper error handling
+   Using a dtmf.in file, be able to send DTMF signals
+   Use a filename specified by the user in the config file
+   Clean up build output so there aren't errors regarding unused vars