Rename multiple columns in an auto migration

[dantownsend]

Fixes #417

If the auto migration manager detected that multiple columns were renamed in a single operation, it could get confused.

For example:

# Old version of table
class TableA(Table):
    a1 = Varchar()
    a2 = Varchar()

# New version of table
class TableA(Table):
    b1 = Varchar()
    b2 = Varchar()

In the above example, a1 could now be b1 or b2. Likewise, a2 could be b1 or b2. Things are quite complex, which is how a bug crept in.