Merge pull request #2 from greydot/master

Add instances and filter messages based on priority.
hsyslog.cabal: tentatively bump the version number to 1.7
Improve the documentation of the internal C functions.
withSyslog: document the fact that the Priority argument has unclear semantics
cabal: update tested-with field
Minor cosmetic changes to System.Posix.Syslog.
Pass a list of priorities to withSyslog. Add logUpTo function similiar to LOG_UPTO() macro in C.