Add format and domain options and support for 100% Jed-compatible output

As I realized in https://github.com/mikeedwards/po2json/pull/15#issuecomment-28869608, the current output is not 100% Jed-compatible. I decided to quickly solve this issue by providing a `format` options, which lets the user to choose if they want a `raw`, or `jed-style` output. 

Because Jed expects the data to be wrapped inside a domain, I also added a domain options, which defaults to "messages" (the same default that Jed uses).

I left the default `format` to `raw` at the moment, but if you think that `jed` is more useful/more often used, then feel free to change it. Also, after you merge this, perhaps its time to tag a release.