change __call__ to _get

we're not going to repeatedly __call__ expecting an updated result
new test

also aliased nose.tools
listing length test
added tests for length and indexing
added length and indexing into object
remove debug message
simplified class

listing now inherits from OrderedDict and is indexable by date, when
indexed, returns a list of items: permanent and freelance listing items,
anything that has a date in parentheses, eg (December 2012)

simplified internally, not passing so many objects around

HNListingItem tuple doesn't have a date anymore
fixed tests to reflect new listing object implementation