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)
fix missing add-zsh-hook in pygmalion.zsh-theme

Fix "command not found: add-zsh-hook" since #3053 removes `autoload -U add-zsh-hook`
Symfony2 plugin use first console found
Symfony2 plugin look for files only
Symfony2 plugin autocomplete for app/console and bin/console
Merge pull request #3087 from hacfi/symfony

Symfony2 plugin use first console found
Merge pull request #2743 from docwhat/issue-2155-scutil

Trust but verify 'scutil' to return ComputerName
Merge pull request #3072 from clippit/patch-1

Fix missing add-zsh-hook in pygmalion.zsh-theme
Support dynamically defined Vagrant machines
Revert "Exit early from git plugin if not in git repo."
Merge pull request #3095 from robbyrussell/revert-1504-master

Revert "Exit early from git plugin if not in git repo." due to #3082
Merge pull request #2744 from docwhat/ostype-not-uname

Use $OSTYPE instead of uname to speed things up
Merge pull request #2811 from stucki/multiple-vagrant-machines

Support dynamically defined Vagrant machines
Add CONTRIBUTING.md file general structure
Categories of issues in oh-my-zsh (work-in-progress)
Format categories of issues in oh-my-zsh (still WIP)

Conflicts:
	CONTRIBUTING.md