Revise os::killtree semantics if process not found.

Change semantics so it is not an error if the process is not found.

Review: https://reviews.apache.org/r/17953
Added missing return value for hashmap::containsValue and test.

Review: https://reviews.apache.org/r/17834
Containerizer (part 1)

The proposed Containerizer interface is to replace the existing
Isolator.

One ContainerizerProcess has been written:
MesosContainerizerProcess - implements containerizeration internally
using a Launcher and one or more Isolators (following review)

The intent is to also support a generic ExternalContainerizerProcess
that can delegate containerizeration by making external calls. Other
Containerizers could interface with specific external containerization
techniques such as Docker or LXC.

Review: https://reviews.apache.org/r/16147
Containerizer - launchers (part 2).

Launcher interface and MesosLauncher to support MesosContainerizers.

Launchers handle the lifecycle of the executor process
(and descendants).

Review: https://reviews.apache.org/r/16149
Containerizer - isolators (part 3).

Isolators perform isolator for the MesosContainerizer.

Isolator interface and implementations of Posix CPU and Mem isolators
(no isolation, just usage())

Review: https://reviews.apache.org/r/16150
Containerizer - cgroup isolators (part 4).

CPU and Memory isolators using Linux cgroups.

This code was written by Chi Zhang <czhang@twitter.com> and based on
the original cgroups_isolator.

Review: https://reviews.apache.org/r/16432
Containerizer - updated tests (part 5).

A few isolator specific tests haven't been updated.

The tests require a different cgroup mount configuration; please see
https://issues.apache.org/jira/browse/MESOS-926 for preliminary
documentation.

Review: https://reviews.apache.org/r/17013
Added a release guide for doing official releases.