fixing dialyzer warnings - w.i.p.
added exometer_report_statsd.erl
dialyzer-reported issues
Correctly configure xref and fix existing xref warnings

As a side-effect the default make target has been changed to compile,
xref check and test the application.
Add folsom to rebar deps to make tests work
Fix relative links and link to overview image
added exometer_slide:new/2
Merge branch 'tb-xref-fix' of github.com:Feuerlabs/exometer into uw-xref-fix
added priv/check_packages.script
merge tb-fix-readme