Drop mention of MongoDB

I don't like the idea of exposing back end tech.
Also Mongo IDs are predictable, i.e. not cryptographically secure.
I am not advocating security by obscurity but telling everyone
about our internals goes just a little too far for me. :)
Also we should retain the option to switch backend storage.
Merge pull request #1 from Sign2Pay/mongo-b-gone

Proposal: drop mention of MongoDB