Assets Library Changes

- Reduced the number of variables used to store directory names for the
library
- Added check for 'assets.directories' config item to set all
directories for the library at once (uses existing config items if not
found)
- trim slashes from the directories for consistent path handling
(assets_url() adds a trailing slash to its returned value)
- Use a single array to hold all of the scripts for the js methods (will
eventually do this for css methods as well, but the desired variable
name ($styles) is already in use by the css methods, so this will
require a more careful modification of the library)
- added setDebug() and setGlobals() methods, deprecated set_globals()
- added $cssMinify and $jsMinify closures to handle minifcation in a way
that will hopefully facilitate replacement of the methods, if needed