Fix e2e

[garbados]

This PR does a lot because I'm a messy bitch, OK?

• Adds new methods like db.importComDB() to make your data more transportable, updates the README accordingly.
• Add examples/e2e.js with a demonstration of doing E2E encryption with pouchdb-adapter-memory.
• Fixed a weird bug about replicating documents created with db.post().
• The encrypted database now stores your encryption key, rather than the decrypted database. No one stores your password!

This is a breaking change.

Closes https://github.com/garbados/comdb/issues/23

[garbados]

Aw yiss