Vastly improve the schema mismatch exception.

[mithrandi]

Fixes #55.

The new exception message looks like this:

exceptions.RuntimeError: Schema mismatch on already-loaded <class 'axiom.test.test_xatop.TestItem'> <'TestItem'> object version 1:
Only on disk:
('bar', 'VARCHAR(64) (this is made up)')
Only in memory:
('bar', 'TEXT COLLATE NOCASE')

[jonathanj]

Wow, that's huge improvement! The code looks good to me, please merge.