Adding sync option on setup page

The details:
- adding radio method for displaying radio button
- adding saving method for saving sync option
- display the interface at setup page
Moving wp_ig() to template-tags file

Template tags have their own place now
Adding sync capability

This is pretty much how it works:

- Upon activation, wp-ig register a (wp) cron task
- A method hooked to that task
- What this method does: a) get latest imported instagram id b) request
using a)’s result as min_timespan c) import the result d) if there’s a
more content, do it in recursive style