Automatically define helper methods for custom BinData types

[y8]

Thanks for this awesome library!

This pull request automatically defines DSL methods for previously defined custom types. This match BinData gem behavior and makes it easier to read binary definitions

Example:

class MacAddress < BinData 
  array octets : UInt8,  length: ->{ 6 }
end 

class DummyPacket < BinData
  mac_address :from
  mac_address :to

  string :text, length: 20
end

[y8]

It's safer to merge this PR after #11 is merged to make sure it passes all checks

[stakach]

Thanks for the pulls @y8 awesome additions!