Merge pull request #3 from SleepyHippo/master

PaaS under the hood, episode 1: kernel namespaces
Rename PaaS under the hood, episode 1: kernel namespaces.md to paas-under-the-hood-episode-1-kernel-namespaces.md
clearify some concepts
Merge pull request #4 from cheneydeng/master

clearify some concepts
Merge pull request #1 from dockboard/master

merge with origin