Add account related metrics

First pass on metrics. @tilgovi confirm this is the right way to go (if you don't mind), and then I'll tackle the others. Also, these aren't one-to-one with @dwhly's list, but should cover the account related ones (+/- queries of the accounts DB). Confirmation on that front would also be fab. :smiley_cat: 

Logs the following things as counts:
h.auth.activate
h.auth.register
h.auth.login
h.auth.reset_password

Partial fix for hypothesis/vision#75