Added a syslog logger

I use libstrophe on a number of embedded linux devices, where syslog is used for generic logging. 
I've created a syslog logger for use in libstrophe, which I believe is of general interest. It has been testet for some days on our setup, only change prior to this pull request is the check for syslog.h, to make it compile on windows platforms (I have no windows machines, and is thus unable to test whether it actually compiles on it).
Feel free to use it as is, or if you have any comments, i'll be glad to look at it.