Support sql.IsolationLevel for transactions

[joeblubaugh]

gorp transactions are created with nil options, which means that all Tx use the driver's default isolation level.

For InnoDB, this level is REPEATABLE READ, which is a pretty loose guarantee.

If there was a method to start a transaction with a given isolation level, my team could avoid doing a fair amount of SELECT ... FOR UPDATE and use the included Get() methods within an isolated transaction.

I know gorp supports optimistic locking with a version column, but we don't use this at my job, and supporting the standard library's options could be nice.

Something like:

func (d *DbMap) BeginOption(option *sql.TxOptions) (*Transaction, error) {

}