Make json the default format

[Toilal]

Maybe it would be better to have {format: 'json'} as the default options object for compile function.

As you have said @gabegorelick in #13, it's really more efficient and natural in gulp environment to use this format as default instead of javascript.

[gabegorelick]

I agree that format: 'json' is vastly superior to format: 'javascript', but I don't want to diverge with what angular-gettext-tools does. This plugin should be as minimal a wrapper around angular-gettext-tools as possible.

[gabegorelick]

FYI there's a PR open upstream that makes the docs a little better in this respect: rubenv/angular-gettext-tools#72