* Add code for use of raw 64 hex-char key in sqlcipher databases.
* Add encrypted db assertion according to sqlcipher doc.
* Add the following PRAGMAS to the API:
    * PRAGMA cipher
    * PRAGMA kdf_iter
    * PRAGMA cipher_page_size
    * PRAGMA rekey

Items are:

  * remove unused imports.
  * __init__.py: initialize variable before read from file.
  * backends/leap_backend.py: Fix mistyped exception name, fix docstring of
    encryption function.

Also does:

  * Remove all crypto methods from LeapDocument.
  * Encode 'encryption_scheme' inside the document JSON.
  * Add functions for encrypting and decrypting.
  * Fix LeapSyncTarget so its connection actually returns a LeapSyncTarget.
  * Fix symmetric encryption when syncing:
    - don't try to encrypt tombstone documents.
  * Fix symmetric decryption when syncing:
    - Only try do decrypt if the incoming document has an '_encryption_scheme'
      entry with value equal to EncryptionSchemes.PUBKEY.
  * Fix doc skipping for non-syncable docs.
  * Fix tests that compared raw content with encrypted content.

This creates a SoledadCrypto object that should encapsulate everything related
to crypto in Soledad.

Also, replace hmac for sha256 when creating hashes.

Tests inherited from u1db still use super, but that can be changed in the
future.