Created mesos::modules namespace for all module related code.

Review: https://reviews.apache.org/r/27020
Checking "kind" before creating a module instance.

Introduced a new kind<T>() function that returns the string
representation of the module "kind". This function is specialized for
each module "kind". Also introduced a templatized
contains<T>(moduleName) that returns true only if a module of name
"moduleName" and kind "T" is available.

Review: https://reviews.apache.org/r/26906
Added sample isolator module.

Created CPU and Memory isolation modules based on Posix CPU/Memory
Isolators. These modules are also hooked up to the relevant Isolator
tests using the typed test mechanism. It also paves the way for future
integration with custom modules specified on the command line.

Review: https://reviews.apache.org/r/26071
Fixed race condition in Latch.

There is a race condition in Latch. The same pid can be terminated by
2 seperate threads simultaneously.

Review: https://reviews.apache.org/r/26783
Use RBT by default and correctly determine if version is >= 0.6

Review: https://reviews.apache.org/r/26420
Differentiate between slave and offer ids

Review:
Fixed missing process.hpp include in zookeeper group.

Zookeeper/group.hpp was missing an include. This was not obvious until we reduced un-necessary includes in libprocess.

Review: https://reviews.apache.org/r/27025