Backward Compatible Support for network >= 2.6

Hi there,

this should give us backward compatible support for network >= 2.6, which means client of network < 2.6 shouldn't be affected.
I was able to test this only in a nix-shell and in a cabal sandbox, but it does seem to work.
Someones willing to test?

I have also bumped the manifest version.
1: I want to fix `iCalendar` in nixpkgs *today* - and that means I have to apply a patch. I think it's ok for a patch to get things building, but I don't think a patch should then switch over to a version that people otherwise wouldn't be able to install outside `nixpkgs`.

2. Not sure there, but I think it's better to cater to the latest package versions, and let older users "opt in" to old support.