Fixed nested 'search-key' used by Morphia for querying MongoDB.

The original String-value was invalidated by previous refactoring
and was not originally caught.
Refactored CommandLineDriver

The main method now gathers CLI arguments and then uses
CommandLineDriver.Builder to create an instance of CommandLineDriver
(which implements Runnable) and then calls its run() method.
Additionally, an issue with the display of usage information was also
resolved (it was a one-liner).
Fixed outdated Morphia package-name from Spring XML
Refactored several classes to account for a change in a method names belonging to RssMediaItem.

The methods in question were RssMediaItem.getUrl() and RssMediaItem.setUrl(String)
which were renamed to RssMediaItem.getUrlString() and RssMediaItem.setUrlString(String),
respectively.