Creates Invoice Settings Controller with views

Settings are no more ing eneral settings. Also I made easier to add logo (You can do it in admin panel). What is more I changed few things about footer_left/right_1/2 - you can setup them in admin panel and its render properly.
These changes make developing this extension a bit easier (it has its own views and controller)

This is invoices settings controller views:
![spree1](https://cloud.githubusercontent.com/assets/4388676/3173801/342ca5f6-ebeb-11e3-9178-f310de95fe29.png)
This is how new footer is being rendered:

![spree2](https://cloud.githubusercontent.com/assets/4388676/3173821/7de1938c-ebeb-11e3-9d4b-e573fc1400b5.png)