Insert select statements

Hi Felipe,
I really like esqueleto, but I was disappointed that it didn't support insert statements. True, you can insert individual records through Persistant, but this doesn't allow you to do things like INSERT INTO table_a SELECT (val_x, val_y) FROM table_b. 

I added this functionality to Esqueleto, trying to keep my changes as self contained as possible. The only caveat is that toRawSql now requires an additional argument, which may break some users' code. 

I hope this functionality can make it into the next version.

Thanks!
Sam