Update README.textile

Added instructions for installing zsh (due to the problems with first time zsh users)
Update README.textile
Update rvm.plugin.zsh

Ruby 2.1.1 -> Ruby 2.1.2.
Merge pull request #2994 from admpsktt/patch-1

Update rvm.plugin.zsh
Merge pull request #2993 from mittalabhas1/master

Update README.textile
Refactor similar functions into common sections
Fix take function

Before the modification, the `take` function will try to cd
into the created directory even if mkdir throws an error.

With this modification, if mkdir fails to create the directory,
cd won't run and the take function will return the mkdir exit
code.