Wrappers for chacha20_poly1305, chacha20_xor

Howdy, 

I should delete the test directory for the arm processor, but you may be interested in taking the addition of wrappers for chacha20_poly1305_encrypt and decrypt, as well as the chacha20_xor stream cipher.  I've added tests for both using the test vectors published at http://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-04

Let me know if you'd if you'd like me to clean anything else up!

Joe