0.15.3 upgrade

[beckjake]

This was one heck of a merge!

This PR is obviously based on @SamKosky's totally awesome work with #46. Where possible, I've prioritized PRs that have merged since it was written over the contents of #46 that did the same thing

There's probably a significant amount of cruft left over (duplicated/vestigial methods and macros, especially). If you notice any in your review, please shout.

I basically can't rebase this, I'm sorry about that. Getting a merge to behave was hard enough!

There might be test failures, I didn't get spark-http running locally.

[jtcohen6]

This looks great.

I just did some end-to-end testing on local Spark and Databricks (including new merge incremental strategy). The only small thing I found is that both the seed and table materializations raise a new-in-0.15 deprecation warning:

* Deprecation Warning: The materialization ("table") did not explicitly return a list
    of relations to add to the cache. By default the target relation will be
    added, but this behavior will be removed in a future version of dbt.

  For more information, see:
  https://docs.getdbt.com/v0.15/docs/creating-new-materializations#section-6-returning-relations

That's something we should resolve before shipping. Otherwise, I think this is good to go.

[beckjake]

I must have lost those in the merge, I think I've fixed it!

[jtcohen6]

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