Added a cache provider for caching to RethinkDB

[Omeryl]

It's missing a few things, but functionally it works.

Things that can be improved upon:

• It could probably do with some more error handling.
• While I store an expiration date, I'm currently not checking it.
• Clearing a cache with a wildcard just flushes the entire cache.
• Flushing flushes everything, not just the specified partition.

[Mark-H]

This probably should just be the standard MODX/xPDO copyright line?

[Omeryl]

I followed the convention, https://github.com/modxcms/xpdo/blob/3.x/src/bootstrap.php#L5

[opengeek]

My concern here is that each partition should be separate and protected from flushes. If this still flushes all partitions when you call it on a specific partition, then I'd like to see that resolved before incorporating this into mainline.