Implement RFC 5424 log levels

[thekid]

http://tools.ietf.org/html/rfc5424 defines the following loglevels

Numerical Code	Severity	XP Logging supports this via
0	Emergency: system is unusable	n/a
1	Alert: action must be taken immediately	n/a
2	Critical: critical conditions	n/a
3	Error: error conditions	LogLevel::ERROR
4	Warning: warning conditions	LogLevel::WARN
5	Notice: normal but significant condition	n/a
6	Informational: informational messages	LogLevel::INFO
7	Debug: debug-level messages	LogLevel::DEBUG

It should be decided whether to implement Notice, Critical, Alert and Emergency.