make observer a setting, need to update docs

overviewer.py

@@ -397,11 +397,7 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
     else:
         dispatch = dispatcher.MultiprocessingDispatcher(
             local_procs=config['processes'])
-    if platform.system() == 'Windows' or not sys.stderr.isatty():
-        obs = observer.LoggingObserver()
-    else:
-        obs = observer.ProgressBarObserver()
-    dispatch.render_all(tilesets, obs)
+    dispatch.render_all(tilesets, config['observer'])
     dispatch.close()
 
     assetMrg.finalize(tilesets)

overviewer_core/settingsDefinition.py

@@ -45,6 +45,7 @@
 
 from settingsValidators import *
 import util
+from observer import ProgressBarObserver, LoggingObserver
 
 # renders is a dictionary mapping strings to dicts. These dicts describe the
 # configuration for that render. Therefore, the validator for 'renders' is set
@@ -93,3 +94,8 @@ processes = Setting(required=True, validator=int, default=-1)
 # memcached is an option, but unless your IO costs are really high, it just
 # ends up adding overhead and isn't worth it.
 memcached_host = Setting(required=False, validator=str, default=None)
+
+if platform.system() == 'Windows' or not sys.stderr.isatty():
+    observer = LoggingObserver()
+else:
+    observer = ProgressBarObserver()