Add support for PostgreSQL 9.2+'s native json type.

[fletom]

The patch keeps things portable by still returning 'text' for unsupported databases/versions.

[bradjasper]

@fletom I'm not very familiar with the PostgreSQL's native JSON type––do you know what effect this would have for people already using a regular text field? How would the migration work?

[fletom]

It should only make a difference when you're creating the field for the first time. The json and text types are functionally almost identical so for existing fields it should continue to function just fine.

[bradjasper]

@fletom Since this is a backwards incompatible change I'm going to save this for the 1.0 release. This should be coming up in the next few weeks.

[fletom]

As I said I think it is actually backwards compatible. Existing fields will continue to work just fine. New fields will just use the json type instead of text, which is fine since they're compatible field type. But go ahead and wait for 1.0 if you still want to.

[bradjasper]

Hey @fletom, sorry I must have misunderstood what you were saying.

I'd like to check this for sure––I've got some time tomorrow when I can look into this. If everything looks good, I'll get this merged.

[bradjasper]

@fletom as I was looking into this today I realized this would potentially collide with another issue we're having (#33)

Basically storing JSON in a string is giving us problems, so we're considering options like prepending "json:" to the string (which wouldn't work with a native JSON field).

We're looking for better solutions, but as of right now this is the only proposal we have. Once we get this issue resolved, I'll jump back on this and see if we can get it merged in.