Add pwiz option to preserve original DB column order.
Hello Charles,

This is a minor modification to enable pwiz to create models that preserve the column ordering from the original source DB. I needed that in order to use dump files created with native Postgresql tools from the original DB. The new behavior an option so that default pwiz behavior is unchanged.  I hope that you will find it to be a useful addition. 

   Ed
 Nice work. Could you add a unit test?
 Hi Charles,

All fixes in. Please let me know if you think the way I've divided up the EXPECTED text is "too clever by half" and if you'd prefer just plain text for the new test. 

  Ed
 > Please let me know if you think the way I've divided up the EXPECTED text is "too clever by half" and if you'd prefer just plain text for the new test.

errr yeah
 I switched to full explicit text for each expected value per your preference. Also prevented test failure of unsupported feature under Python 2.6
 Thank you so much! I've mergd a very slightly modified version of your changes. I also caught an unreported bug while messing with the tests, so even better!

Add pwiz option to preserve original DB column order in generated model definitions.  Very useful when a DB created by Peewee will be loaded from a dump file created with native DB tools (e.g. Postgresql INSERT type dump).
