Category excluding filter.

This filtering appender allows you to choose some category
names that won't be logged to the delegated appender. This
is useful if you have e.g. a category that you use to log
web requests to one file, but want to keep those entries
out of the main log file without having to explicitly list
all the other categories that you _do_ want to include.

Has one option, "exclude", which is a category name or
array of category names. The child appender is set in
"appender", modelled on the logLevelFilter.
Merge pull request #119 from UniversityofWarwick/category-filter

Category excluding filter.