wrong module name in fast_counter default
Add 'ad_hoc' exometer_entry type

Normally, exometer_entry types must be made known to exometer
before entries of such types can be created. However, if a few odd
metrics need a different definition, it is awkward to first have to
define a template for that metric, and then create it.

The ad_hoc type requires that `module' and `type' are specified
in the Options list. The `type' option may be parameterized, e.g.
`{histogram, Arg}', which will be treated the same as if a known
type had been parameterized in that way (i.e. `{type_arg, Arg}'
is added to the Options list). See test/exometer_tests.erl for an
example of how to use.
Fix exometer_folsom:get_value/4

Added exometer_util:get_statistics/3, which is a lightweight
version (with slight differences) of bear:get_statistics_subset/2.
Change exometer_histogram to use exometer_slide.

The slot_slide version of the histogram currently tends to discard
too many extreme values, essentially only showing the 'min' and
'max' of the averages from each slot.

Also added some EUnit tests.