Making auto-correction off by default

- Allows for the user to turn on auto-correction using the
$ENABLE_CORRECTION variable
- Adds aliases regardless of variable assignment to aid users that use
setopt to turn correction back on in their zshrc
Merge pull request #2316 from BBonifield/master

Making auto-correction off by default.

Okay, I don't feel so strongly these days about this.
Cancel update if the current user doesn't have write permissions for the oh-my-zsh directory.
Merge pull request #2358 from n-st/master

Cancel update if the current user doesn't have write permissions for the oh-my-zsh directory.
Revert "Cancel update if the current user doesn't have write permissions for the oh-my-zsh directory."

This reverts commit 152e1e07e0397ecc3f1778ea597d081c33ee8ff4.