Fixed str and add bugs in bandit data containers

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @suntzu86 

********* DESCRIPTION **************
Branch Name: norases_387_fix_bandit_bugs_and_add_new_tests
Ticket(s)/Issue(s): Closes #387  

********* TESTING DONE *************
make test
make style-test
Added two tests for SampleArm.__add__. (testing add armsHistoricalData also implies testing this).