Fixes a graphite key formating bug.

* Fixed a bug where having both API key and prefix empty was giving
  a metric key starting with a dot.
Merge pull request #39 from sylane/fix_empty_api_key_and_prefix

Fixes a graphite key formating bug.
Fix formatting of warning message
Merge pull request #42 from ates/fix-warn-msg

Fix formatting of warning message
Fix formatting of datapoints

* Push percentiles to Graphite
Merge pull request #43 from dcorbacho/graphite

Fix formatting of datapoints
Fix reset of probes
Merge pull request #46 from dcorbacho/uniform

Fix reset of probes
support percentiles in exometer_report
Fix uniform percentiles
Fix probe delete

* Give mod_state to the probe callback
Avoid division by zero in stats computation
Merge pull request #47 from dcorbacho/uniform

Fix uniform percentiles
Fix probe handle_msg
Merge pull request #48 from dcorbacho/probe-del

Fix probe delete
Allow {apply, {M, F, A}} in static subscribers list

The result of apply(M,F,A) must be a list of legal subscriber
tuples (including possibly {apply, {M,F,A}}.
Added {select, Expr} pattern for predefined subscriptions.

Also, added a start phase for predefined reporters.
The reporters must start *after* the predefined entries have
been registered, in order to avoid a race condition.