Add ability to sort contents by date published in table

Addresses https://github.com/nbproject/nbproject/issues/54

Currently displays "Date Added" in table with the ability to sort by clicking the header.

If we want to add the ability to sort by date viewed, I'm guessing we don't want that information in the table (too much clutter). Would it make sense to have a line above the table that says "Sort by: last seen, date added, alphabetical" where you can just click the words to sort? Should it default to sorting by last seen?