Trust but verify 'scutil' to return ComputerName

Apparently, it is possible to set up a Mac such that
`scutil --get ComputerName` hasn't been set.

This change checks if that fails and falls back to the original
mechanism.

Closes #2155
Closes #2183
Use $OSTYPE instead of uname to speed things up

The $OSTYPE variable is set at ZSH compile time and can be safely used
to determine the OS of the system. e.g. darwin (os x)
WP-CLI plugin init
Add README.md for adb-autocompletion plugin.
Improve support for Vagrant VM definitions
Use the library function for ruby prompt

Not everyone uses rvm and the library has already got a function for this.
fix missing add-zsh-hook in pygmalion.zsh-theme

Fix "command not found: add-zsh-hook" since #3053 removes `autoload -U add-zsh-hook`
add git difftool alias (gdt)
add wp-cli plugin readme
Merge pull request #3069 from johnjohndoe/feature/adb-autocompletion

Add README.md for adb-autocompletion plugin.
Revert "Add a CONTRIBUTING file to instruct people on issues and pull requests"
Merge pull request #3076 from robbyrussell/revert-2766-add-contributing-file

Revert "Add a CONTRIBUTING file to instruct people on issues and pull re...
Revert "Add Composer's local binaries to PATH"
Merge pull request #3077 from robbyrussell/revert-3068-patch-1

Revert "Add Composer's local binaries to PATH"
Merge pull request #3070 from stucki/vagrant-vm-list-fix

Improve support for Vagrant VM definitions
Merge pull request #3071 from halilim/patch-4

Use the library function for ruby prompt
Merge pull request #2996 from joshmedeski/master

WP-CLI plugin init
Merge pull request #3073 from ajhenriques/master

add git difftool alias (gdt)
Symfony2 plugin use first console found
Symfony2 plugin look for files only