Disable grid by default.
Fixes grid's tile cache.
Still beyond sub-optimal, but at least it works.. sort of.
Re-runs go-bindata to include convert.js
Expands code comment in world.go.
Adds some code comments to world.go
Finetunes ImageType api. These constants define the type of
overlay to be rendered. These are specified through the
-overlay command line parameter, which can also be set to
'all'. Which is a shortcut to selecting all known overlays.

Fixes ImageType.String() to correctly account for bit flag
values.

Moves ImageType code to a separate file. It is getting a
bit big to be mixed in with ImageMap code.