Allow queries to be specified as a dict.

Adds the ability for the queries parameters to be defined as a mapping
of zoom level to the query.  Undefined zoom levels are assumed to be null.

This makes it a little easier to quickly see which query goes with which zoom level, without counting all the nulls.  Zoom levels with no query do not need to be defined.  

The internal representation of the queries array is untouched.

The example in server.py:
```
"queries":
              [
                null, null, null, null, null,
                null, null, null, null, null,
                "SELECT way AS __geometry__, highway, name FROM planet_osm_line -- zoom 10+ ",
                "http://example.com/query-z11.pgsql",
                "query-z12-plus.pgsql"
              ]
```

may be specified equivalently as:
```
"queries": 
              {
                "10": "SELECT way AS __geometry__, highway, name FROM planet_osm -- zoom 10+ ",
                "11": "http://example.com/query-z11.pgsql",
                "12": "query-z12-plus.pgsql"
              }
```