Include information about recent changes.

The checkbox wasn't getting filled in correctly. By some overzealous and
uncaught copy-pasting, I had wrapped the field in the function to
populate a select box. Since this is a checkbox, we want to use its
`.checked` property instead.

Use a timeout also. By default Time Task marks Billable as true/checked.
If the script unchecks the box and then the AJAX select box population
occurs, the check box re-checks itself. Adding a timeout allows it to
remain unchecked if you've set that in the user options.

Make it a little clearer what's going on in the date picker event
handler.

Include some background information about the extension, installation
instructions, and release information.

Query the Chrome storage API to get user options for time entry
fields. Replace our hard-coded values with user-entered ones.

This allows the extension to work for others without requiring
modification of the source code.

In order to be able to pull the options, I had to move the main
extension code into a callback function that can be passed to
`chrome.storage.sync.get`.

Allow a little more breathing room for input content. One option was too
large for the input, which is why I'm increasing the width.

I had copied this line directly from the Chrome Extension Options page
documentation, but it looks like it has a typo. Fix the syntax error.