refactor all of the fieldmap/select code into its own file to clean up sqlx.go a bit
attempt at #45, select into *[]byte, which is safe but involves a copy;  unfortunately RawBytes cannot be used in Row.Scan
use *interface instead of *[]byte, which works for all types
add Unsafe() to DB, Tx, Stmt, make it inheritable when preparing, etc, change reflect stuff to return the old error
add some description of Unsafe to the readme
fix a small code example in the readme