Ensuring to properly restore and instantiate Column Selector

- Column settings service updated to provide a slightly more elaborate column id but ensuring it is not unique so that it can be referenced and matched when restoring saved settings
- In settings controller watch of "spreadsheet.url" value, it now conditionally checks the oldValue to ensure it was not undefined or an empty string before resetting columns to []. It would be undefined or "" if this is the initializing phase of the settings app. This conditional is to avoid clobbering previously saved column settings when restoring saved settings
Merge pull request #23 from stulees/master

Ensuring to properly restore and instantiate Column Selector