Add a command for generating CSV for Google Fusion Tables

It's often useful to be able to easily visualize a particular
subset of areas from MapIt on a map.  This command
(mapit_make_fusion_csv) lets you specify a particular area
type and generate a CSV file that can be imported into
Google Fusion Tables to show all areas of the that type
randomly coloured.  You can also restrict this to only show
areas of that type that are covered by another area.

Some instructions for importing the CSV into Google Fusion
Tables is given in the comments at the top of the file
and I will create a similar page on code.mapit.mysociety.org
Allow use of JSONP when redirecting from a code

This fixes issue #49.  There are two changes introduced in
this commit:

 - In the customized area lookup for country 'gb', pass on
   the query string if returning a redirect response.

 - In the JSONPMiddleware, only deal with the callback
   parameter if the response isn't a 302 redirect.
Add an 'intersects' query type

This new query type finds any areas that intersect the given
area.