engine/views:

Removed a decorator which was reinventing the wheel and replaced it
with a django built-in equivalent (for limiting request types to views
and returning a 405 if the request type--POST, GET, etc.-- is not
supported).
engine/views:

Return 404s (instead of empty data sets) where there is no calculation
or result data.
engine/views:

Pyflakes cleanup.
engine/views:

Instead of a 501, return a 404 if an export type is not supported.
Merge pull request #8 from larsbutler/add-404s

Add 404 responses in the case of non-existent artifacts

[r=matley] [f=*oq-platform-1216832]