Implement JSON configuration file

Implement JSON configuration file

A JSON configuration file can now be read as "/etc/
pulseaudio-agl.cfg", and a sample one is provided by the
package. If it exists and is valid, it will override the
default configuration.

As a result, the module now depends on json-c.

Change-Id: I3f5deccc6f9b89cbd54bea7004271b8aec727538
Signed-off-by: Manuel Bachmann <manuel.bachmann@iot.bzh>