Improved HTML output of sidebarlogo and simplfied user interface

These are the most important changes and the reason for each change:
1. Changed application of stylesheets: Old version added a style-tag into the body which works in most browsers but violates HTML rules. New version now uses the style attribute instead or uses id or classes from the theme stylesheet. This method is preferred over inline styles.
2. Added an alt-tag option for the image since this is required by HTML.
3. Made only the most basic field visible by default. This is true for backend, where advanced options are now placed in groups as well as the frontend where only title, logo and description are visible by default. The backend contains a group for all the styles needed and a group for the advanced fields as well as sorting of all elements inside the plugin. This makes the plugin easier to start with while preserving the full functionality.

I believe that these changes provides a good compromise between required features and complexity. Different to HTML nugget it already provides the basic formatting and elements of a common sidebar item (title, description block). With the option to provide a logo, it is definitely sufficient to provide an about box many blogs use. While more advanced options are possible, the average user is not overwhelmed by a large set of options. This is especially true after dropping the style-tag, thereby creating the need for a style configuration field for each extra element present in the plugin. Yet all the options are available.

Oliver