Fix descriptor signatures and keys so that Stem can verify them.

 * ADD dependency on pycrypto==2.6.1.
 * CHANGE the way all keys are generated.
 * FIX PKCS#1 encoding.
 * CHANGE all strings to bytes.
 * ADD several new strings to leekspin.const.
 * FIX several issues with various descriptor digests.
 * ADD support for creating relay descriptors (warning: probably buggy).
 * FIX encoding of fingerprints and identity digests.
 * FIXES #13045 https://bugs.torproject.org/13045
Merge branch 'fix/13045' into develop