standalone crypto

[j0sh]

removes dependency on openssl; we only need a few primitives. axtls has a favorable license but its sha-1 implementation would need to be converted to sha-256.

[lu-zero]

ffmpeg does have sha256 libavutil/sha.c if you plan to use it for some more stuff it might be useful.

[j0sh]

pros:

• was planning on using ffmpeg for rtp anyway

cons:

• it is substituting one dependency for another at this point
• no way of generating diffie-hellman keys in ffmpeg. not needed immediately so it is something we can deal with later. axtls doesn't have it either. but rtmfp/rtmpe will need those when/if they are implemented.
• also rc4 is needed in the case of rtmpe, but i am disinclined to support that when rtmps is a better alternative.

keeping openssl might be the best way to go in the long-term now that i think of it.