Removed setLoadingMessage() method

As I said in the issue I created, the activity that displays the activity circle (loading spinner) should simply display the activity circle without any text. Merging this pull request will ensure that this library adheres to the Android design guidelines. 

For reference: http://developer.android.com/design/building-blocks/progress.html#activity

EDIT: I also went ahead and added a javadoc comment with the list of authors that contributed to the DynamicBox class.