Stub storage for map configurations (#114)

TODO:

 - Garbage-collect stored map configurations
Require grainstore 0.17.0 to avoid leaving redis keys around
Use redis EXPIRE command for garbage collection

As a side-effect changes map configuration keys back to flat:
map_cfg|<token>
first implementation for torque renderer
Merge branch 'maps-api' of github.com:Vizzuality/Windshaft into maps-api
We don't need to run grainstore GC during testing
Set mapconfig expiration time using backward compatible directive

The directive is grainstore_opts.default_layergroup_ttl
documented public methods
updated doc in torque module
Merge branch 'maps-api' of github.com:Vizzuality/Windshaft into maps-api
Make server use of "renderer" object format-agnostic

The renderer format is initialized at renderer creation time
Stop talking about "getGrid", as the renderer interface is only getTile now
Document second parameter passed to getRenderer torque callback
Have makeRenderer return either a Torque or a Mapnik renderer, based on format
Draft documentation for generic renderer factory
Renderer factory API: rename "getRenderer" to "createRenderer", add "disposeRenderer"
Use an adapter for TorqueRenderer, to expose a close() method
Add routing for fetching torque tiles
fixed torque error format
Merge branch 'maps-api' of github.com:Vizzuality/Windshaft into maps-api