Add to and pretty up AverageOverTime's report

Also, since it keeps a total anyway, it's kind of rendered Total
redundant so we'll leave it out of the call to #register
actually better if we just stick to returning numbers here, even if they have a lot of decimal places
'subject' syntax reads a little better here
Change to tracker initialization

We may not always need to provide an #initialize, but we can probably
always expect that #start should be called and will do the pre-run
initialization. Constructor arguments would serve other purposes, to be
elucidated shortly.
First passing TopHashtags spec
Use Faker to make up fake hashtags in tests
Top hashtags tracker
Add TopHashtags to the executable