added exometer_report_lager.erl

The exometer_report_lager reporter is a slight modification of the tty reporter. It does the same kind of formatting, but emits a lager log entry (at a configurable level) instead of printing to the tty.

Note that lager doesn't allow custom log levels, so one of the existing log levels must be used. The default is 'notice', which is the level below 'debug'.