Muhhshoaib add new tab in instructor dashboard ex 11

@chrisndodge i have implemented the business logic for the CRUD Operations in the Instructor Dashbord. Once the design is completed then i'll update the changes into design. I have attached some snapshots that indicate where i have placed the E-commerce tab. These changes are not final, it is only for review.

Thanks
![screenshot from 2014-06-13 19 41 11](https://cloud.githubusercontent.com/assets/7291787/3271291/774089b8-f30a-11e3-947b-ceecb871695d.png)
![screenshot from 2014-06-13 19 41 18](https://cloud.githubusercontent.com/assets/7291787/3271292/77436854-f30a-11e3-8628-e5581f8e8342.png)
![screenshot from 2014-06-13 19 41 30](https://cloud.githubusercontent.com/assets/7291787/3271293/7743865e-f30a-11e3-8d40-024f1750fb80.png)



The unused argument here is course_key_string.  Why is it an argument if it is unused?