Specify a windshaft SHA in package.json

The specified SHA is version 0.13.1

This allows us to update the master branch of our fork of Windshaft
without automatically triggering an update to the tiler.
Merge pull request #19 from jwalgran/topic/specify-windshaft-sha

Specify a windshaft SHA in package.json
Typo
Add support for filtering on udfs

Filtering on string udfs is now support using the following syntax:

```
{'plot.udf:Nickname': ...}
```

The main goal is to intercept udf field names and transform:

```
plot.udf:Nickname -> "treemap_plot"."udf_scalar_values"->'Nickname'
```
Merge pull request #20 from azavea/topic/udf-filtering

Add support for filtering on udfs
Add cluster support

Take advantage of multiple CPUs/cores.
Remove lint and normalize whitespace in server.js
Merge pull request #21 from jwalgran/topic/cluster

Add cluster support
Don't try to escape null values
Merge pull request #22 from maurizi/fix/nullvalues

Don't try to escape null values
Make tiler server params optional
make redis host customizable
Change cache_buster url argument

Presumably fixes https://github.com/azavea/OTM2/issues/659 but I could
not repoduce the issue, which was only shown in production with load
balanced tile servers.

Adam was able to make this bug disappear by adding a render_cache.clear
to every tile request. Searching the Windshaft code, I found the tile
renderer caching respects a query string argument named cache_buster

https://github.com/azavea/Windshaft/blob/master/lib/windshaft/render_cache.js#L236
Merge pull request #26 from jwalgran/fix/cache-buster

Change cache_buster url argument
Plot is now a subclass of MapFeature

In preparation for supporting other types of map features:
* Update tiler SQL to be MapFeature-centric rather than Plot-centric
* Update "plot" to "mapFeature" in variable/function names

But don't change the app/tiler interface yet, to keep things simpler until we actually support new feature types.
Merge pull request #29 from RickMohr/topic/plot-is-a-map-feature

Plot is now a subclass of MapFeature
Add jshint configuration file
Fix some typos and bad code style
Continue further in the transition from plot -> mapfeature

Note that this *will* break search queries for properties on plot
(but we don't have any in OTM2)

It is intended to allow searching by tablename in addition to model name
with a later change
Get feature_type in the base and UTF Grid requests for use in styling