A layer to the srtp driver test is added to be able to run the tests both in-place & not-in-place without duplicating the test.

The not-in-place functions are currently a minimal implementation using a memcpy.

The protect/unprotect api's can now operate both in-place or not in-place, depending on the requirements of the caller.
The length of the out buffer can now be checked to ensure there is sufficient space.

Tests are add to verify validation of the output buffer length.

Also update the minimal code example in README.md to reflect the new
api.

With the new api the full length of dst is now passed in, if it is too small it should fail.

This is a safety check to ensure we do not wrap dst len when removing tag size.
All of this code would get much cleaner if the tag could just be returned instead of cached. see cisco#714