Bugfix - logger now has default formatter object at construction
async_sink: catch back thread exceptions and rethrow on the calling thread on the next logging request
patch async_sink
Merge branch 'master' of https://github.com/gabime/spdlog
fixed gcc warning about unordered intialization
Bug fix in registry
static_cast<int> to remove warning
Remove warning in debian about unused variadic template args
Fixed warning
Update README.md
Add NOTICE, ALERT and EMERG log level.

This commit introduces 3 new log level. Thoses are:
    + NOTICE, which is a bit worse that INFO, but still not a warn.
    + ALERT, for case worse that critical.
    + EMERG, application is unusable.

With those 3 log levels, spdlog now has all log level accepted by
the syslog() system call.