Fixed str and add bugs in bandit data containers

********* PEOPLE *************
Primary reviewer: @sc932 

Reviewers: @suntzu86  @Feriority

********* 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
you should also verify that arm1 now has the expected value. like
```
arm3 = arm1 + arm2
arm1 += arm2
check ids
check arm3 equals arm1
```