Don't create single-used prepared statements for Exec inside a txn
DbMap.Exec - remove commented code (related to #141)
Merge of #127 from @dvander
error instead of panicing in tableFor
#138 - fix spelling of services
Added types for Null* from database/sql

Added types for Null* from database/sql and changed "double" to "real" for postgres val.Name() switch.
#121 - cleanup old Nullable* strings. Use "double precision" for float64 with postgres.