Prepend instead of append to path

When an activity is launched, an import should first search the activity
bundle source rather than /usr/bin

Fixes the need to change activity sources every time a system package
conflicts with an import.

May impact any activity that relies on a system package being in search
path before the bundle source.  Forty activities were tested, and none
relied on this.

bin/sugar-activity

@@ -114,7 +114,7 @@ def main():
     os.environ['SUGAR_BUNDLE_PATH'] = os.path.abspath(os.curdir)
 
     bundle_path = os.environ['SUGAR_BUNDLE_PATH']
-    sys.path.append(bundle_path)
+    sys.path.insert(0, bundle_path)
 
     try:
         bundle = ActivityBundle(bundle_path)