added SIP Testing Modules with a SIP Library

I developed a SIP library with 5 demo modules for easy SIP module development.
github.com/fozavci/gamasec-sipmodules repo includes a few basic usage.
VulnVoIP (it's a vulnerable SIP server distribution) could be used to test sample modules.
SIP Library has many SIP features, but I'll plan to extend it.
I will add custom header support, full invite support, SDP support and 
fuzzing support to library. Also a simple SIP proxy could be developed with it.