Minor tweaks

* Make upgrade validation secure (using filters was opening it up to security risks)
* Lower curl timeouts for faster file size retrieval / errorring out
* Only add js localization if we actually use js
Prevent double adding of styles if/when main function is called several times
Updated readme with filter extension requirement and upped version nrs
Apply option validation best practice

Hook validation routine into sanitize_option rather than use register_setting as this will ensure that validation will run on both front and back-end. This is mainly important for the upgrade routine and simplifies that code.

Also better naming of one method.
Readme tweak
Update links to WP repo
Merge pull request #13 from jrfnl/master

Version 3.2.1