message classification with liblognorm sample code

I have just enhanced liblognorm's normalizer tool to support the -t option. If it is given, only messages with the specified tag will be output. Currently, only a single tag can be specified. The main purpose of this change is to provide some example code on how to use the message classification API, so that other developers can include it into their solutions more easily.

In essence, the whole logic is contained in normalizer.c, line 122 and 123. The application needs to keep the "wanted" tags inside an es_str_t type. Then, it needs to call the ee_getEventField() API to find out if the normalizer (better said: its rule) associated the tag with a given message. That's it...

Please note that we may implement a more powerful API in the future -- if this makes sense. If you think API additions would be useful, please suggest them together with a description of the benefits.