Make retrieval of blocks in ajax request defensive

If one of the blocks it attempts to retrieve is missing, the thing
would throw an exception. This commit makes it check that a block
exists before calling the toHtml() method on it.
Add specification of category blocks by config.xml

Makes the extension more extendible, to allow custom blocks to be ajax
layered by the frontend.

NOTE: This doesn't update the JS so you need to do that yourself in your
site if you want it to work. We need to overhaul the JS anyway. The
extension continues to work as shipped, ajax loading the layered nav and
the product list blocks onto the page.

Additionally, we apply a few small fixes to make the whole thing more
defensive, and in one case deleted a for loop that was never doing
anything.
Update search results controller to work like category

Takes its list of blocks from config.xml and returns them, if found.
Move frontend/default/... to frontend/base/...

Also replaced package.xml with modman.