Fix #111: Rename LogLevel enums

[zenonparker]

As described in Issue #111

[mfontanini]

Thanks for this. This will break people's code but it won't work as it is so we have no choice. @accelerated heads up, not sure if you're using this outside of cppkafka.

[accelerated]

Well the alternative non-breaking solution would have been to #undef LOG_EMERG & others right above the LogLevel enum. That being said, I prefer the CamelCase enum names even if they're breaking code.

[mfontanini]

Right but then if you have an application that uses both syslog and cppkafka, then you'll have an issue when trying to use those macros.