Invalid projection

Using a tif downloaded from http://www.gisat.cz/content/en/products/data-download, omnivore is unable to determine the projection of this file.

```
digest ~/tmp/strange-projection.tif

/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:78
var current = new mapnik.Projection(this.projection);
^
Error: failed to initialize projection with: ''
at Raster.getExtent (/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:78:17)
at Raster.getCenter (/Users/r/gh/mapbox/mapnik-omnivore/lib/raster.js:67:8)
at /Users/r/gh/mapbox/mapnik-omnivore/index.js:80:12
at pop (/Users/r/gh/mapbox/mapnik-omnivore/node_modules/queue-async/queue.js:24:14)
at /Users/r/gh/mapbox/mapnik-omnivore/node_modules/queue-async/queue.js:38:39
at /Users/r/gh/mapbox/mapnik-omnivore/index.js:75:7
at Object.oncomplete (fs.js:108:15)
```

`gdalinfo` gives the projection information as:

```
LOCAL_CS["KRON D211",
UNIT["metre",1,
AUTHORITY["EPSG","9001"]]]
Origin = (-907000.000000000000000,-933000.000000000000000)
Pixel Size = (250.000000000000000,-250.000000000000000)
```

cc @GretaCB @bsudekum

Start by reproducing the failure with the referenced TIFF and `gdalinfo`, then read `lib/raster.js` around line 78, where the empty projection reaches `mapnik.Projection`. Trace how the projection is obtained before `Raster.getExtent` is called. Done means this input no longer fails while determining its metadata and center.

Written by the indexing model from the issue text.