Replace PyCrypto with cryptography.

Implement some functions for handling random generation using os.urandom
minor fix against zipfile infos
Merge pull request #213 from globaleaks/feature/cryptography

Feature/cryptography
removed safe umask concept
Add flag for making it possible to force an encrypted only node.
Make ramdisk usage optional.

Required for OSX.
Expose encrypted_only via the node interface.
Add migration script from 11 to 12
added some unit test on encrypted_only and no keys