make sure .db is initialized at start of profiles init

fixes
File "aqt/profiles.pyc", line 253, in _loadMeta
  File "aqt/profiles.pyc", line 234, in recover
AttributeError: 'ProfileManager' object has no attribute 'db'

aqt/profiles.py

@@ -63,6 +63,7 @@ class ProfileManager(object):
 
     def __init__(self, base=None, profile=None):
         self.name = None
+        self.db = None
         # instantiate base folder
         if base:
             self.base = os.path.abspath(base)