Adding 'alias-field' method for segments

This PR adds the ```alias-field``` functionality which is required to fix #14 .

The method ```alias-field``` would work like this:
```ruby
class MockSegment < HL7::Message::Segment
   weight 1
   add_field :no_block
   alias_field :no_block_alias, :no_block
   add_field :validating do |value|
     value == "bad" ? nil : value
   end
end

msg = MockSegment.new(@base)
msg.no_block        #output => "no_block"
msg.no_block_alias  #output => "no_block"
```

Hey @rojotek, @Stratus3D, @jgn Would you mind taking a look at this?