fix #1

[sergioramos]

• allow to receive a level instance instead of creating it
• always instantiate Model's when it gets values from the database
• implement Model.get.all (returns a stream) and Model.remove.all (removes using a stream).
• implement tests (with coverage)
• increment docs
• rename to a comply to level naming conventions

I would also like to apply to help maintain so that I don't keep my fork :)

[sergioramos]

updated the PR: https://github.com/ramitos/level-modella/commit/df7628525b183e42bae1b7a4448f97579f2dfdff