allow periods in jsonp callback function name

- now it plays nice with angularjs jsonp calls
- Reference: https://github.com/WP-API/WP-API/issues/144#issuecomment-46325232

rmccue responded with "Interesting; it was actually intentional to leave out . (not _) to avoid the possibility of side effects, but I guess I'm OK with adding it." but nothing has been added yet, at least I couldn't find a branch with the changes.

I am currently using WP-API with an Angular frontend, and I needed to make this fix, so I thought I would contribute it back as well.