Revert "And so there are no need for merge_grids function"

This reverts commit 57b0ee08ce4c48e38c91175bfd4b21a4314afb66.
Using built-in set instead of deprecated sets.Set
Bumped to 1.39.0 with addition of TileStache.Goodies.ExternalConfigServer and fix to Mapnik Grid
Added tentative new Blit module with one working unit test
Masks and second unit test now work
Colors and two more unit tests now work
Color alpha and fifth unit test now work
Added five alpha-compositing tests
Externalized Blit library, started core Sandwich provider to replace Composite
Added support for blend modes in Sandwich provider
Importing all of Blit in one namespace
Added support for adjustments in Sandwich provider
Added zoom ranges to layers
Extracted most of Sandwich provider logic from renderTile() to a new function
Added Sandwich as a provider supported by name alone
Adapted Composite documentation to Sandwich provider
Added Sandwich provider to built documentation
Importing Sandwich provider only when needed
Pushing internal layer configuration to a static prepareKeywordArgs() method of provider classes, starting with Proxy, UrlTemplate and Sandwich providers
Added static prepareKeywordArgs() method to Mapnik provider classes