Added another missing feature bit.
The asset loop now uses the correct subobject of the root.

Before, it believed the array to be the root. That used to be the case,
but the spec has changed!
Added plugin support! Albeit with a deprecated standard library feature.

Python v3.3 supported the necessary operation (loading an arbitrary .py
file) but that bit has been deprecated as well. To my knowledge, there
is no clean or simple way to do this with Python v3.4 without a lot of
string handling or messing with sys.path extensively, which I would
rather not do. We'll see how it turns out...
The assets.json can now specify a distribution root!
JSON syntax errors don't inform the user that the file wasn't found anymore!