Extract plot search by geometry from view into a model manager

I want to make use of this search by geometry logic in the API so I moved it
to the Plot model.
Add locations API endpoint to allow searching for plots near a point

Requests take the form of /api/v0.1/locations/39.956825,-75.1588311/plots

In addition to adding the endpoint I made a few refactorings in this commit:

- I modified the api_call decorator to properly pass through args and kwargs

- I moved the conversion of a list of Plots to a list of dictionaries into
  a shared helper so it can be used by multiple view functions.
Merge branch 'v1.2' of github.com:azavea/OpenTreeMap into v1.2
Merge remote-tracking branch 'origin/v1.2' into v1.2
Merge branch 'v1.1' into v1.2
fix new api endpoint
review #53, add in tree photo end points
remove extra url comments and document raw api decorator
Use the more efficient .count() method when counting plots

.count() triggers a COUNT(*) query rather than len() which triggers a request
for all the object data.
add in binary point packing
Merge remote-tracking branch 'origin/v1.2' into v1.2
add in signing
http basic auth
fix django dumb headers
wrong key used for authorization
Merge branch 'v1.1' into v1.2
fix auth tests
reset password
image api
better route handling