Added definitions for the Important and Warning admonitions in the
formatting section.
Removed the caution admonition from the formatting conventions section
Add Tango icons and use them for admonition graphics aswell as some
xsl and css to make them look better.
Removed the table border from the manual navigation
git-svn-id: svn://localhost/ardour2/trunk@1458 d708f5d6-7413-0410-9779-e7cbd77b26cf
