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.
Hi Ollie, by points:

1. Sure thing, even though I thought bumping the version would have been a better choice. Granted, we might need to issue a PR against nikpgs, but wouldn't you need to do the same to revert the "broken = True" on this package anyway? Just curiosity then: what's the rationale between the non-bump?

2. I left it to False to preserve the "backward-compatible" spirit: it will continue working untouched for who has network < 2.6. Make sense?