Upgrades windshaft (and grainstore) to be able to specify the tile
format, see: https://github.com/mapnik/mapnik/wiki/OutputFormats
Merge pull request #201 from CartoDB/CDB-3664

Upgrades windshaft (and grainstore) to be able to specify the tile format
Prepares next release
Release 1.14.0
Merge tag '1.14.0' into cdb