Modified Document class to support folders in course documents.
Document now uses the Builder pattern to construct.
Modified DocumentsLoader to support loading Documents in sub
directories.
Added some directories to the test data.
Added a generic file icon to Documents in the DocumentsFragment list
view.
Added drawable for icon.
Added file icons for word, text, and powerpoint types.
Increased size of Document's ListView empty text.
Added icons for more types of files.
Sort Documents list view to show folders above files.
Increase padding between drawable and text in Documents list item.