Zsh completion and small typofix

Hey Tom,                                                                        
                                                                                
After trying out your stupid tool installer today for the first time (which is really not so stupid at all :P), I've added an autocomplete script for our favorite `$SHELL`.     
                                                                                
I don't exactly know how you would like to install it, so I leave that up to you, if you decide to pull this in.                                                               
                                                                                
There is a Vim modeline at the bottom, not just to nag you, but primarily to enforce spacing according to the Zsh completion style guide (https://github.com/zsh-users/zsh/blob/master/Etc/completion-style-guide).                                                            
                                                                                
I've also added two small typo fixes in the README.md                           
                                                                                
Cheers,                                                                         
                                                                                
Benjamin (happy sti user)