Fixes, enhancements, and a very little cleanup.

Increased CSS z-index for thickbox.
Added proper support for SSL verification.
The API key verification on the settings page will now catch response
failures and display an error.
Added cURL detection to notify user if cURL is not installed.
Added a way to hide the donation box in case it gets in the way.
Changed default caching value to "0".
Added method to handle version update changes. (I didn't need it until
now.)
Improved admin notice transient.
Added a screenshot of the settings page.
Increased minimum WordPress version requirement from 3.1 to 3.2.
Made additional minor changes.