clean up the command structure for dbt test

[jthandy]

few concerns:

• the more common thing to do will actually be to run dbt test --validate rather than dbt test. it's strange that the more common use case is the one where you need to supply a flag.
• "test" vs "validate" as verbs don't have super-clear meanings. we should think for a bit here about what we want these terms to be prior to really launching testing and building docs around this stuff.
• when i use dbt test --validate i would really prefer that it not do the part where it builds the "limit 0" temporary tables. i would imagine running this validation once a day or even multiple times a day, on already-working dbt models. there is no need to build these test models in this environment because we're actually validating the real dbt models.

[drewbanin]

i rolled this out as a part of 0.4.1

# run models
dbt run
# create "limit 0" views and delete them afterwards. model names are prefixed with "test_"
dbt run --dry
# run schema validations from schema.yml files
dbt test