add register_json

[cswingle]

Adds register_json to the Connection and Pool classes. Essentially copied from the register_hstore code.

[haizaar]

Thanks.

[cswingle]

Thanks for merging. I probably should have bumped the version number, etc., so that pip and friends would know whether they're getting a version with or without it.

[haizaar]

Well, from time to time we tag new version and update PyPi. Lets give your code some time and we'll push it then. I prefer not to update version before then.

[cswingle]

Lets give your code some time and we'll push it then. I prefer not to update version before then.

Fair enough. Thanks.