Add support for SRTP

[doe300]

SRTP (Secure Real Time Transport Protocol) is a profile for the RTP protocol.

Thus, it should be possible to include it into our program by extending the RTPPackageHandler and providing additional classes for the encrytion/decryption and authentication/verification.

See: https://tools.ietf.org/html/rfc3711 and RFC 4771

NOTE: The complete use of STRP should be made optional with flag in CMakeLists.txt to skip (which results in skipping checking and downloading crypto-libraries)

[doe300]

Add support to SIP: http://tools.ietf.org/html/rfc4568
Optional extensions:

• Add encryption for headers: http://tools.ietf.org/html/rfc6904