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.
I see the point about 1, make sense.

I don't mind defaulting the flag to True, even though the Cabal constrain solver is smart enough to enable/disable it to try new install plans. I will amend shortly so you can fix iCalendar today.