Add UTFGrid movement support

This is really just a sketch of how it would work. In particular, it
provides mechanics for adding a new interactive layer and relevant
callback.

So far the callback is sort of useless.
Merge pull request #13 from ahinz/topic/utfgrid

Add UTFGrid movement support
Add scope_model method to Instance

Each instance can filter down any model with an instance field down
to records from that instance. The recommended usage is to apply
this method on the instance that is hanging off of a request.

If an model doesn't have an instance field, Django would raise a
FieldError, just like when using the ORM.
Merge pull request #15 from steventlamb/topic/scope_model_to_instance

Add scope_model method to Instance
Plot is the basic unit of viewing, not trees
Provide a hash method based on audit fields

Since each audit record effectively provides a revision id, we can
simply use that as a simple hasher
Provide a more complete hash for plots

By default the audit hash does not provide for foreign keys that
reference the given object. For instance if we had the following:

Tree(id, plot_id)
Plot(id)

Updating the tree's plot with "tree.plot = someoplot" would update the
hash for the tree but not the plot record.

Since we want the plot's hash to change whenever a tree is updated at
all we use the tree's hashes as well as the plots.
Refactor templates

Maps js was being pulled into all of the pages, even when it wasn't needed.
Merge pull request #17 from ahinz/topic/etags

Add etags to basic models
Add in OL default stylesheet
Show XYZ layers correctly with Bing maps
Provide a default layer for TMS
Merge pull request #18 from ahinz/fix/bing

Bing maps are weird
Add Role, FieldPermission and Instance.default_role
Add Authorizable mixin, rework object heirarchy

Add model mixin Authorizable to handle user authorization of
model level permissions. Model has methods for validating
a user's ability to edit a field.

Factor out UserTrackable class from Auditable to also be used
by Authorizable.

Add Authorizable inheritance to Plot and Tree.
Add tests for Role, FieldPermission, Authorizable

Update permissions in existing tests and refactor setUp
to be more DRY.
Merge pull request #21 from steventlamb/topic/role_models_initial

Topic/role models initial