Normalize error return for post_to_solr to be {error, _}

Standardizing tagged error returns to be {error, _} makes it easier to
write consistent generic error handling code.
Add doc and specs, rename init_add_items => init_items

post_single and post_multi return ok instead of skip now when passed
no-op input.

Fixed add_item to properly update rather than overwrite the context
object.

Added delete_item for completeness (even though it isn't needed for
reindexing).