Added support for getting extensions from a X509Req object.

Currently, one can put extensions into a X509Req object but has no way to get
back those extensions. This change adds the Python code needed to do this.
This change depends on the addition of CFFI bindings for
X509_REQ_get_extensions.
Finished test case for test_crypto.py.

Finished the test case that called for OpenSSL.crypto.X509Req.get_extensions
to be implemented.
Merge remote-tracking branch 'upstream/master'
Fix test cases in test_add_extensions
Merge branch 'master' of https://github.com/pyca/pyopenssl
Made corrections as per suggestions in
https://github.com/pyca/pyopenssl/pull/22
ChangeLog entry
Drop the underscore prefix on the other local variable name.
rstify the docstring a little more
Drop the redundant starting value for `range`
rst api docs
Merge pull request #51 from pyca/sholsapp-master

Introduce `X509Req.get_extensions`