ARC4 support
add ARC4 keysize testing
Merge branch 'master' into arc4-support

* master:
  Typo
  Be really explicit about what's good and bad
  Mention return types.
  Consistently use e.g.
  Module documentation.
  Single space.
  Add a new Mode interface to document mode.name and start on some prose docs for interfaces.
  Proper name for the iv thing.
  Actually note the properties for cipher modes types on their ABCs.

Conflicts:
	docs/hazmat/primitives/symmetric-encryption.rst
Fixed two bugs in the PKCS7 padding where unpadding would accept bad inputs.
Merge pull request #238 from alex/padding-only-fixes

Fixed two bugs in the PKCS7 padding where unpadding would accept bad inputs
update docs to include arc4 example
Merge pull request #214 from reaperhulk/arc4-support

ARC4 Support