Replace settings to options; move css files from content/ to skin/; rename something

1. Because "options.xul" is the default name for inline options.

   https://developer.mozilla.org/en-US/Add-ons/Inline_Options

2. Skin is more correct place for our stylesheets.

   https://developer.mozilla.org/en/docs/Chrome_Registration

3. Like "nodes" to "elements", "attribute" to "attributeName".

   No any important.
Add attributeValue property for Tweaks.js
Add basic debug mode to extension
Change type of tabsBorder tweak to attribute
Add cssPath property for Tweaks.js