Add info log level, with precedence between debug and warning.

D1 supports the 'info' log level and there was a couple of mentions in
D2 documentation regarding an 'info' log level. Adds support to
Role::Logger and an 'info' keyword to the DSL for logging info messages.
Update POD with details of info level logging.
Merge branch 'Pr/loglevel_info' of https://github.com/veryrusty/Dancer2 into veryrusty-Pr/loglevel_info
Update changes
Merge branch 'veryrusty-Pr/loglevel_info' into devel