Stop django-pipeline from hiding globally defined functions

There is some Javascript code inlined in <script> elements that relies
on calling functions that are defined globally in Javascript files that
are being handled by django-pipeline. However, by default
django-pipeline wraps each Javascript source file in an IIFE, and then
yui-compressor correctly thinks it's free to obfuscate the names within,
so those functions are no longer available globally.

We could fix this either by declaring each such function with:

  window.some_function = function () { ... }

... or by supressing this behaviour entirely by setting
PIPELINE_DISABLE_WRAPPER = True.  Since there seem to be several such
examples, and the fix is urgent, this commit takes the latter approach.

(An example function which would not be found without one of these fixes
is add_marker_to_map in pombola/core/static/js/map.js.)