Fix for issue #593: Glyphicons fail to render + minor: avoid a PHP notice

--- Fix for issue #593: Glyphicons fail to render

The culprit is @icon-font-path, a relative path defined in variables.less. Its modification by the compiler and the fact that the CSS output is put elsewhere both make the path invalid, in the end. In class-SS_Framework_Bootstrap.php, I've defined a Less variable named @assets-url which contains the absolute URL to the assets folder. @assets-url is now used in variables.less to determine @icon-font-path

--- Minor: avoid a PHP notice if WP_Widget_Recent_Comments is inactive

In cleanup.php, check if WP_Widget_Recent_Comments is present before referring to it.