Support for RFC 5424-style packets and extended runes in syslog tag via AddAllowedRunes() function

[chrissnell]

This PR adds two new features to the package:

• Support for RFC 5424-style packets
• Support for runes within the syslog tag that would otherwise be RFC non-compliant. These runes can be specified with the AddAllowedRunes() function, which takes a string of runes that shall be allowed:

    s := syslog.NewServer()
    s.AddAllowedRunes("-._")  \\ Allows dashes, periods, and underscores in the syslog tag
    s.AddHandler(newHandler())