Add a quick icon for the toolbar (Just modified the executeCode.png file)
Extended the current execution options to include the ability to execute the code between two sections.
This is similar to the execute code block present in Matlab (such a great feature)
In the event that the cursor is not in a valid code section nothing will happen.
I have also added an icon to the menu bar in highlighting this new functionality.
It would be nice if the icon and the menu option were only present when and if the cursor is in a valid section block.
change command treatment for run section

- Cmd+Alt+S as keyboard shortcut
- Not on editor toolbar
handle executing section with null end (last section in document)
Merge pull request #81 from rstudio/justacec-justacec/CodeBlock

Add run section command