Allow defining enum value mapping

[julik]

At the moment state_machine_enum relies on Postgres enums. The values for the enum attribute are collected from the collector, but there is no way to map them either to specific integers (closed: 0, open: 1) or to strings (closed: "CLOSED", open: "OPEN") as they are picked up from the Collector. At the moment the only mapping we support is from string to itself - closed: "closed", open: "open"} We need a way to make this customisable - one way could be like this:

state_machine_enum :foo do |sm|
    sm.value_map { |enum| enum_sym.to_s.upcase }
end

or similar. Or we need to add a test that the passed mapping overrides the one received from the Collector and there is a check if there is a discrepancy (come to think of it - this might even be better, as the standard Rails way of defining an enum can then be preserved)