Merge pull request #890 from JonRowe/enable_warnings

Configure warnings behaviour with --warnings
Show warnings
Show the slowest example groups along with the slowest examples in a report when using `--profile COUNT` (default 10).
Cosmetic changes.
Add example group's filename and line number in the output.
Use the same style as for the examples.
Treat the same doc string in different files as different example groups.
Merge pull request #887 from lfu/master

Show the slowest described classes along with the slowest examples in a report when using `--profile COUNT`
Merge pull request #884 from JonRowe/make_reporter_register_and_send

Send formatters only messages they implement
Clone rather than dup Time, fixes #894
work around issues with dup/clone
refactor redefinition of time
update changelog