Optional 'delta' Argument in runAnalysis API

I updated titanic.py's API runAnalysis to make 'delta' an optional argument with default value of 7.  I also updated the README file and test_titanic.py to note this change.

I apologize for the excessive number of commits.  This is my first time using GitHub, so I had to fiddle with it for a while to get it to do what I wanted.

Let me know what you think!  Thanks!