GridFS support

[apocolipse]

This is for #90, Adding gridfs support.

Sorry i'm not a ruby dev so maybe there's some better way to handle this.
I found Sequel couldn't properly use encode_bytea (which is what was needed to get around encoding issues mentioned in the issue ticket), so added in pg as a requirement for that, if you'd like to reduce dependencies you should find how to get Sequel to offer encode_bytea as a class method, I could only get it to work as an instance method by passing the pg object down

Also didn't have time to write tests (aside from finally successfully loading my gridfs store, which houses pdfs/ word docs/ images/ plaintext/etc) also i'm not overly used to ruby's test's.
I did /run/ the current tests and all but one passed (for $timestamp, but it was failing on master also so i'm gonna say not caused by this)

[nelhage]

Thanks for this! I'd love to have at least some basic tests, though – is there anyone else on your team who might be able to help you out? I might be able to find some time to point you in the right direction, if not.

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