Feature/new materializations

[drewbanin]

Summary:

This branch implements ephemeral and incremental models.

Usage:

models:
  package_name:
    events:
      materialized: ephemeral 
    sessions:
      materialized: incremental
      sql_where: "collector_tstamp > (select max(collector_tstamp) from {{this}})"

Description:

Ephemeral models will be injected as CTEs into any model that ref()erences them. Ephemeral models are part of the dependency graph and generally function like any other model, except ephemeral models are not compiled to their own files or directly created in the database.

Incremental models will create a new table if the table doesn't already exist, or insert new records into the existing table if it does already exist. New records are determined by the sql_where field. The {{this}} parameter points to the model name, in this case sessions.

[jthandy]

@drewbanin just want to make sure that, prior to pushing this to pypi, your release notes include the fact that this is a breaking change and you let folks know how to update their dbt_project.yml files. I mean--this doesn't say it, but I think you changed true/false to be table/view, right?