Allow for back button in App bar

[yhbrandon]

Following material standards, a need for a back button in the app bar is fairly common. The use of a custom icon used to alleviate this problem, however this has been removed in the latest update. I have extended out the default functionality to allow a back button to be shown based on props. Not sure what exactly the strategy is regarding in app navigation.

[mmrtnz]

Thanks for bringing this up @yhbrandon! It seems that we forgot to update the icon prop for AppBar during the transition to using FontIcon. The new replacement prop for icon is iconClassNameLeft. There are also two new props which allow you to add other React Elements on the left and right sides of AppBar. More info about them can be found in the commit's comments here.

[yhbrandon]

Thanks!