Validate the IV/nonce length for a given algorithm.

Fixes #159
Implement this for CTR
Tests for OFB and CFB
test for ctr
flake8
fixed typo
Merge branch 'master' into validate-iv

Conflicts:
	tests/hazmat/primitives/test_block.py
Merge branch 'master' into validate-iv
Merge branch 'master' into validate-iv

Conflicts:
	cryptography/hazmat/primitives/ciphers/modes.py
	tests/hazmat/primitives/test_block.py
So the tests don't all explode
Merge branch 'master' into validate-iv

Conflicts:
	tests/hazmat/backends/test_openssl.py
	tests/hazmat/primitives/test_block.py
Explanatory comment
Merge branch 'master' into validate-iv

Conflicts:
	cryptography/hazmat/primitives/ciphers/modes.py
Merge pull request #272 from alex/validate-iv

Validate the IV/nonce length for a given algorithm.