Add geodjango support to Measurement model

Use the geodjango objects manager in the
Measurement model. This is needed to be
able to do geographic queries through to
the point foreign key.
First commit for matchups code

First go at finding matchups between
in-situ data and images.

At the moment, it just does a search for
each image then returns the JSON results
in a list.
Constrict matchup to single measurment type

Allow user to specify a particular measurement
type to search for in the in-situ data. (Passed
though in the "parameters" field in the json
file) as with other options.
Add ordering to measurement api

Allow measurements to be ordered by point
field with all relations, so we can order
results by time
Update docs

Update the matchup code's docstrings,
and add to the documentation
Merge branch 'matchups'