CFB support

This requires a bit of explanation. OpenSSL has methods that implement
standard CFB, 1-bit CFB (cfb1), and 8-bit CFB (cfb8). Unfortunately,
while old (read: 0.9.7) versions of OpenSSL appear to test these
variants, newer versions have a comment stating that cfb{1,8} are
unsupported. Accordingly, I've backed out any support for the variants
for now. We can add it back into the CFB class if and when we gain a
backend that supports arbitrary s for 1 <= s <= block_size
add CFB to documentation
Merge pull request #59 from reaperhulk/cfb-support

CFB support
OFB is specified as having an IV instead of a Nonce.

https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Output_feedback_.28OFB.29
ModeWithNonce is no longer used.
ModeWithNonce no longer exists.
OFB uses an initialization vector instead a nonce.
Use the version of PyPy from PPAs to get PyPy 2.1
Merge pull request #62 from dstufft/test-pypy

Enable testing on PyPy
Merge pull request #60 from dreid/ofb-iv-not-nonce

OFB uses an IV instead of a nonce.
API refactor to separate cffi declarations

* Moved to structure similar to OpenTLS
* api modified to load the cffi modules
* cffi verify methods broken out
* Jarret Raim and I pair programmed this
Various PR review fixes

* Changed some single quotes to double quotes
* Moved the files in the cffi package back into the openssl package
  * evp, opensslv
* Changed attrs from ALL CAPS
* Fixed up docstrings
* Stopped using fromlist=['*']
* No fallback on definition imports. You must supply includes, types,
  functions
* Change includes, types, functions to be strings rather than lists for
  now
* Removed teardown since we're not using it right now
remove import function, rename attrs to upper in modules
Merge pull request #67 from reaperhulk/api-refactor

API refactor to separate cffi declarations
Slightly clean up the OpenSSL binding

 - reduce the amount of state needed on the API instance
 - fixed import ordering
Inline this function, it didn't do much
Merge pull request #68 from alex/cleanup

Slightly clean up the OpenSSL binding