best constant for all loss functions

This pull request adds a functionality of getting best constant and best constant loss values for all loss functions currently supported by VW. Code is moved to a separate file (best_constant.cc) to keep main.cc small.
More details can be found here:
https://github.com/JohnLangford/vowpal_wabbit/issues/406#issuecomment-63278712

Pls note that there are no any changes to VS project files or makefiles. So it will break VW build process as main.cc will be dependant on best_constant.cc till someone add best_constant.cc to list of project files  for compilation. Unfortunately I have no VS to make these changes and include them in pull request. 
I think that line 601 is never reached because there is a default value "squared" defined at
https://github.com/JohnLangford/vowpal_wabbit/blob/master/vowpalwabbit/parse_args.cc#L563