Add a transient flag to structs managed by gorp.

Allows you to mark fields as transient without using "-" as their name. This allows you to select into those fields, but does not attempt to write those fields back to the database. Useful for loading relationships.

Example: 
```golang
type WithTransientColumn struct { 
   Id         int64
   Created    int64
   OtherField int64 `db:"other_field_name,transient"`
}
```

This also provides a framework for parsing flags that can be used for #26 and similar features.