Add alwaysUpsert flag

Adding this flag based on the documentation here:
https://github.com/mongodb/docs/commit/238d6755a74c3c978cc272d318283f726379a43c.

For backwards compatibility setting the default to alwaysUpsert = false.
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Note that the always upsert flag is only avaiabile in mongo 2.6+
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Merge pull request #27 from Clever/flag-for-upsert

Add alwaysUpsert flag