caching: Use separate caches for config variants and other contexts. * The `_cache` directory is now organized in multiple "sub-caches" for different contexts. * A new context is created when config variants or overrides are applied. * `serve` context uses a different context that the other commends, to prevent the `bake` command's output from messing up the preview server (e.g. with how asset URLs are generated differently between the two). * Fix a few places where the cache directory was referenced directly.

--- a/piecrust/environment.py	Sun May 03 23:45:32 2015 -0700
+++ b/piecrust/environment.py	Sun May 03 23:59:46 2015 -0700
@@ -137,7 +137,9 @@
         self.exec_info_stack.clear()
         self.was_cache_cleaned = False
         self.base_asset_url_format = '%uri%'
+        self._onSubCacheDirChanged(app)
 
+    def _onSubCacheDirChanged(self, app):
         for name, repo in self.fs_caches.items():
             cache = app.cache.getCache(name)
             repo.fs_cache = cache