Initial support for multilayer api
Split multilayer testing from the main one
Add support for fetching tiles by token
Add support for fetching utfgrid by multilayer token

Needed another entry in the cache renderer key.
Require application/json data on layergroup post

Add tests for bogus calls handling
Change layergroup POST response to match specs (token -> layergroupid)
Add afterLayergroupCreate hook called on layer creation
Merge branch 'multilayer'

Conflicts:
	lib/windshaft/render_cache.js
	lib/windshaft/server.js
	test/unit/render_cache.test.js
Target 0.9.0 (for multilayer API)