whitespace tweaks
make constructor more flexible

* now apiCallback is optional as you may not need to set it at all in cases
  when you are purely making request using the access token
* getLoginUrl() now takes an array of options including previous scope array and apiCallback
* use http_build_query() for safer url building
* getApiCallback() now throws InstagramException if a callback uri hasn't been set yet (through __construct(), getLoginUrl(), or setApiCallback())
* updated documentation to reflect changes