network: add iptables rules to explicitly allow forwarding

Explicitly enable container networking for Fedora and other distros that
have a REJECT all rule at the end of their FORWARD table.
Make "docker insert" errors obvious

Closes #1130
See also #1942
Add ImageInsert tests
Update release script with proper support for non-*.docker.io bucket URLs
Update release script to move https://get.docker.io/ubuntu/info to https://get.docker.io/ubuntu/ and provide a backwards-compatibility redirect (same for /builds/info)
prevent deletion if image is used by a running container
- Added delete all the containers example
Fix the display of get.docker.io in Firefox by making our index files text/plain
- Updated description
Edits after code review

Return long image ID
Return streamformatted error or "raw" error
CONTRIBUTING: wrap a couple of long lines

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Update contrib/vagrant-docker with projects available
Make sure docker kill exit with failure in case of error
Just return the id; nothing else.
Merge pull request #2641 from artagnon/master

CONTRIBUTING: wrap a couple of long lines
Merge pull request #2646 from fgrehm/contrib-vagrant

Update contrib/vagrant-docker with projects available
use utils.ParseRepositoryTag instead of strings.split in docker images
add wait, stop, start, restart, rm, rmi
Merge pull request #2621 from rogaha/rm_del_all_example

- Added delete all the containers example (docker rm `docker ps -a -q`)
Merge pull request #2527 from jpoimboe/fedora-iptables-fix

Fix container networking on Fedora