work around a bunch of bugs;

dataset/persistence/database.py

@@ -134,7 +134,7 @@ class Database(object):
         """
         Get a listing of all tables that exist in the database.
         """
-        return self._tables.keys()
+        return list(self._tables.keys())
 
     def __contains__(self, member):
         return member in self.tables

dataset/persistence/table.py

@@ -253,6 +253,8 @@ class Table(object):
 
             table.drop_column('created_at')
         """
+        if self.database.engine.dialect.name == 'sqlite':
+            raise NotImplementedError("SQLite does not support dropping columns.")
         self._check_dropped()
         self.database._acquire()
         try: