Sclark 384 blog ab example

********* PEOPLE *************
Primary reviewer: @suntzu86 

Reviewers:

********* DESCRIPTION **************
Branch Name: sclark_384_blog_ab_example
Ticket(s)/Issue(s): Closes #384 

********* TESTING DONE *************
manually run
make test
make style-test
i believe your blog says epsilon was 0.15

this should also be a named constant at the top