Merge: Add and improve warnings
* Add tags (labels) for warnings. could be used by Jenkins or other tools to group warnings
* Add second-level warnings, called "advice", used to limit warning inflation a la java
* Add new warnings
* missing doc
* unread local variable
* useless empty init
* useless attribute type <- was already present, but made a advice, instead of a warnings.
Close #722
Pull-Request: #759
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>