Adds formatting to get_last_granule and replaces fill value with Empty

`get_last_granule` attempts to stringify values from recent granules of a
dataset. Currently the values are displayed as-is, this patch replaces
fill values with the word "Empty" to signify there is no value there and
it formats time stamps into ISO8601 Zulu-time.

<img src='http://f.cl.ly/items/3m0I28211L0D0U3v1W2J/Screen%20Shot%202013-03-12%20at%2010.55.01%20.png' alt='Example'></img>