Refs #472 Add metadata for Custom Events reports (for scheduled reports, piwik mobile)

plugins/Events/API.php

plugins/Events/Events.php

plugins/Events/lang/en.json

tests/PHPUnit/Integration/ApiGetReportMetadata_yearTest.php

tests/PHPUnit/Integration/CustomEventsTest.php

...ts_Events.getAction_lastN__API.getProcessedReport_day.xml

..._Events.getCategory_lastN__API.getProcessedReport_day.xml

...ents_Events.getName_lastN__API.getProcessedReport_day.xml

...Events_eventCategoryOrNameMatch__Events.getAction_day.xml

...ents_eventCategoryOrNameMatch__Events.getCategory_day.xml

...omEvents_eventCategoryOrNameMatch__Events.getName_day.xml

...egmentMatchesEventActionPlay__Actions.getPageUrls_day.xml

...s_segmentMatchesEventActionPlay__Events.getAction_day.xml

...segmentMatchesEventActionPlay__Events.getCategory_day.xml

...nts_segmentMatchesEventActionPlay__Events.getName_day.xml

tests/PHPUnit/UI