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