Builder: extract the execution of a single build step into buildFile.BuildStep

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
missing quote quota

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
improve the Vagrant VMs

This bumps the VM memory and corrects forwarded ports when they're used
already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
iptables: use dest_addr and dest_port for public port FORWARD rule

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3919 from SvenDowideit/missing-quote

missing quote quota
Merge pull request #3922 from jpoimboe/iptables-public-dest-port-fix

iptables: use dest_addr and dest_port for public port FORWARD rule
Merge pull request #3557 from unclejack/improve_vagrantfile

improve the Vagrant VMs: bump RAM to 512 MB & correct forwarded ports
builder: clearly display ONBUILD triggers during a build.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #3254 from shykes/onbuild

New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build
docs: Installation on Mac OS X 10.6 Snow Leopard
Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)