Updated docs and doc strings, removed outdated docs.

This reflects my current understanding, but please let me know if I'm mistaken.  Much of this is repetitive, but I think it's important to make sure people know there is an easy way to get-or-create a metric.

Note: removed following from meters.rst documentation: 

The second argument to ``meter`` is a string describing the "units" for the
meter.  In this example it's "files", as in "18732 files".

as it doesn't seem accurate from reading the code.


Note: removed following from histograms.rst documentation:

You can create an unbiased histogram by passing an extra boolean argument
(though you probably don't want to)::

(def search-results-returned-biased
     (histogram reg "search-results-returned-unbiased" false))

as it doesn't seem accurate from reading the code.