Lightweight asset mapping

Introduces a lightweight mechanism for unilaterally mapping assets by
defining appropriately named constants. Can be used, for example, to 
map jQuery to a cdn by defining  ASSET_MAP_ALL_FILE_JQUERYJS as the cdn 
path. Can also be used to resolve duplicated assets by mapping all to 
a common file and package.

Successful mapping obviously depends on packages using the html 
helper correctly.