Added examples of a simple and an advanced script
Save the Quick Script when it's executed

- The last executed Quick Script is saved and reloaded in the textarea
when opening the popup later on.
Bug correction and refactor

- The "Delete script" feature now works.
- Added a callback parameter to saveScript and deleteScript so the
options page is reloaded on changes.
- Refactored the code to access the user_scripts object in
chrome.storage.local more easily (doSomethingWithScripts => saveScript,
deleteScript).