Refactoring and class separation

Hi there, 

I've used the ruby-hl7 gem in the past and it was very helpful for me.  I wanted to add a new features but I found out that the spec wasn't working on ruby 1.9.3.  I've spent some time refactoring and I'd like to get feedback about the changes I've made.

Here is a list of changes:

- Added travis.yml for ruby 1.9.3, jruby and rubinius
- Upgraded rake, rspec and replaced rcov with simplecov due ruby 1.9.3. compatibilities.
- Upgraded rake file to be compatible with the updated versions of rspec and rake.  I haven't test all the rake task on ruby 1.9.3
- Extracted the Date, Time and DateTime methods from the ruby-hl7.rb file into a date_time.rb file.
- Extracted the Message class form the ruby-hl7.rb file.
- Created a MessageParser fro the methods related to parsing from the Message class.
- Extracted the Segment class from the ruby-hl7.rb file.
- Extracted a SegmentGenerator class from the Message class.
- Extracted a module SegmentListStorage from the Segment class with the methods related to child segments (nested segments).
- Created a spec_helper