Add upsert support to Model.save, Document.save, and Document.saveAll

[mstade]

This adds a second (optional) parameter to Model.save, Document.save,
and Document.saveAll; which allows you to specify how conflicts should be
handled when inserting documents.

When options.conflict is set to either update or replace, documents
will be updated or replaced when inserted if they already exist in the
database. If the value is set to anything else it will revert to error.

The default conflict resolution is error, which is in line with how it
worked before since that's what RethinkDB defaults to.

[marshall007]

It would also be great to have these same options on Model.saveAll().

[mstade]

Agreed @marshall007, I'm being bitten pretty hard by that at the moment. I'll see if I can get that working as well.

[mstade]

Ok so I hacked something in for Document.save and Document.saveAll as well. I'm not too pleased with this however, since it requires you to specify null for modelsToSave when calling Document.saveAll; i.e.:

myDocument.saveAll(null, { conflict: 'replace' })

Ain't pretty.

[neumino]

Err I'm really sorry, I missed this pull request -_-

I'll add options for Model.save but I'm a bit more worried about Document.save because of the joined documents. I'll try to merge this, but since I've refactored Document.save, this seems hard.

Again, really sorry @mstade for missing your pull request :/

[xpepermint]

+1 (Document.save)

[mstade]

@neumino Don't worry about it buddy, it's easily done in all the noise! Since publishing this PR I've moved on to other things and am no longer actively using either RethinkDB and thus not thinky either. At the time, missing upsert was a big deal(tm) and a pretty big gotcha. Whether this is still true with current versions of thinky I wouldn't know, so you may want to consider just closing this PR or creating a new issue to deal with this for a more up-to-date code base.

Finally – thanks for your great efforts to bring better tools to the developer community!