add AES CTR support to OpenSSL 0.9.8
add changelog entry
allocate only 16 bytes

cffi allocates 17 bytes (one for trailing null), but that isn't
used by this function so we don't need to allocate it.
add some comments to explain the existence of AESCTRCipherContext
add test to verify AES CTR is always available in 0.9.8+, comment updates
rename all the things if you want them to work
Merge pull request #1068 from reaperhulk/ctr-mode-098

AES CTR Mode 0.9.8 Support