add auto reconnect in execute_sql
Because `OperationalError` might signify other things than just a closed connection, I'm not too excited about merging this.  Perhaps you can subclass your `Database` class and add this logic yourself?
 Check out 51c66e7ed6bcc56940eb39d30c6d9333290e9b55 (and the subsequent commits).  You can now define a `sql_error_handler` on your database subclass, which should hopefully help solve your problem.
 In case you're interested, I've got a mixin for retrying failed queries in 017e4e4952f0b429dd21f0e90465a0f644cf6015 . Docs:

http://docs.peewee-orm.com/en/latest/peewee/database.html#automatic-reconnect

I use peewee with tornado, and all requests share one database connection, but when something wrong happens with database, the connection will be unavaliable. So I added a `try exception` block in `execute_sql()`, and this works fine for me.
