Add TypeMap support

[icholy]

This lets you use nullable types without hard-coding them. Here's how you use it:

dbmap := &gorp.DbMap{Db: db, Dialect: gorp.PostgresDialect{
	TypeMap: map[reflect.Type]string{
		reflect.TypeOf(MyInt64{}):   "bigint",
		reflect.TypeOf(MyFloat64{}): "double precision",
		reflect.TypeOf(MyTime{}):    "bigint",
	},
}}

Addresses #348 #334

[icholy]

I just found SqlTyper