added new "except" filter functions

The new `filter_except` and `regfilter_except` functions are identical
to their `filter` and `regfilter` counterparts but keep parameters that
would have otherwise been filtered out.

Fixes #7
readme: merged the return value in the prototype

This is what Varnish 4 does with the generated RST documentation, it
also looks less cluttered.
updated copyright years in relevant files