Fixed Issue #5531 -- Re-arranged placement of items in keyboard shortcut

Closes https://github.com/ipython/ipython/issues/5531. 

Users were confused on how to use keyboard shortcuts because the pop-up
window documentation was confusing. This commit refactors the keyboard
shortcut JS files; the shortcut information is now arranged in a clearer way.

Major code changes: 
1. Switched from dictionary structure to an array of dictionaries to store the keyboard shortcuts info.
2. Removed the variable `help_index` completely; removed the `help.sort` function. 

BEFORE: 
![keyboard_shortcuts_original](https://cloud.githubusercontent.com/assets/954858/2703636/d0ff4ef4-c456-11e3-8b25-ba27d425c661.png)

AFTER:
![keyboard_shortcuts_v1](https://cloud.githubusercontent.com/assets/954858/2703637/d6d5ee0a-c456-11e3-9616-56dc9466e059.png)