Merge Phil Zimmermann's libzrtp as a FreeSWITCH library

Thanks to Phil Zimmermann for the code and for the license exception
we needed to include it.

There remains some build system integration work to be done before
this code will build properly in the FreeSWITCH tree.

libs/libzrtp/projects/gnu/create_docs.sh

@@ -0,0 +1,11 @@
+cd ../../doc
+rm -f docs.tar.gz
+rm -rf libzrtp-doc
+echo "=================> start doxygen."
+doxygen > /dev/null 2>&1
+mkdir libzrtp-doc
+cp -Rf ./out/html/* ./libzrtp-doc
+tar -zcvf ./libzrtp-doc.tar.gz ./libzrtp-doc >> /dev/null
+rm -rf libzrtp-doc
+mv libzrtp-doc.tar.gz ../projects/gnu
+cd ../projects/gnu