API Change: changed default local assets dir to 'root/assets'

Changed default local assets dir from 'root/src.d' to 'root/assets'.
This can be changed via new 'Model::RapidApp' config param
'local_assets_dir' and disabled altogether by setting
'auto_local_assets' to false.

Also added 3rd local auto asset 'Icons'. To recap, by default, assets
will be setup for each of the following dirs if they exist in the local
app:

 * 'root/assets/css'
 * 'root/assets/js'
 * 'root/assets/icons'