Add no-op default recipe to sprout-osx-base.

In order to make sprout work on ubuntu machines, we need to load sprout-osx-base attributes. All the recipes in this cookbook are osx specific, but some of the attributes are needed for non-osx recipes, so a no-op default.rb would allow the attrs to be loaded.
Merge pull request #238 from patcon/patch-1

Add no-op default recipe to sprout-osx-base