dialyzer fixes, changed newentry callback

NOTE: Incompatible change!

* Dialyzer warnings fixed.
* The exometer_report callback `newentry/2` changed to `newentry(Name, Type, Opts)`. This is an incompatible change, but done to avoid leakage of the `#exometer_entry{}` record.

Users who have their own reporter implementations will have to change their code. A simple fix is to use `exometer:info(Name, entry)` to fetch the `#exometer_entry{}` record.