Feature/materialize incremental

[drewbanin]

Materialize models as incremental. Each execution of dbt run will

1. create the model if it doesn't exist
2. insert new records (via the model definition) into the model

To take advantage of this feature, update your dbt_project.yml file:

models:
  'JamJar':
    events:
      materialized: incremental                 # set this field to 'incremental'
      sql_field: cast(collector_tstamp as date) # specify a field name or arbitrary sql
      ...

Incremental models can also built with sort and dist keys (specified in the config)

cc @jthandy