Add ability to set custom font

[pivotal-csaa-dev]

Adds a block that can be set on any instance of BNRDynamicTypeManager which will allow the user to set a custom font (in the event that they have their own custom accessibility fonts, as described in http://stackoverflow.com/questions/20510094/how-to-use-a-custom-font-with-dynamic-text-sizes-in-ios7)

[humblehacker]

This change seems very useful. Is there anything that we can do to get this pull request merged?