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
Include a long description in our setup.py
Also use an absolute path for the about file
add to author list
Merge pull request #397 from alex/long-description

Include a long description in our setup.py
backend support check now lists which backend caused the skip
Merge pull request #399 from reaperhulk/ego-stroking

Add to author list
Merge pull request #398 from reaperhulk/backend-skip-backend-name

Show which backend caused a skip