Ignore build/ folder and _cffi__*'s

Former can come from python setup.py bdist_wheel
Latter can come from python setup.py develop
Merge pull request #695 from Ivoz/cffi-ignore

Ignore build/ folder and _cffi__*'s
Merge pull request #675 from reaperhulk/rsa-verification

RSA Verification Support
add RSA verification support
Add PSS support to RSA sign/verify
some kwargs for pss tests
review comments from previous PR applied to this one
add a check for an error I can't reproduce outside the CI infra
even more terrible debugging
c'mon
...
flailin'