This Fixes #1126 : showing cell numbers of parts

1> Created a div element and appended cell id to it in cell_view.js
2> Added style required for this div element and set z-index to 4, so that it can appear above the inner div , in edit.css
3> Adding a new cell from the prompt cell, below the existing cell creates  a sequentially numbered cell below it
4> Adding a new cell in between two cells renames the cells below the newly added cell
5> Moving the cell 1 over the cell 2, renames the cell 1 as 3 and similar behavior is observed further

screenshots attached after testing : 
![add_new_cell_in _between](https://cloud.githubusercontent.com/assets/6388509/5556211/09066960-8cfa-11e4-964b-1ca8d0f5044f.PNG)

![moving_1st_cell_over_2nd](https://cloud.githubusercontent.com/assets/6388509/5556213/090b5cae-8cfa-11e4-9175-e779db74ef18.PNG)

![numbering](https://cloud.githubusercontent.com/assets/6388509/5556212/0906f880-8cfa-11e4-855d-ee7736653a09.PNG)

@gordonwoodhull 
Please review and let me know if this is fine.