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.
Merge pull request #15 from robbyrussell/master

merge from mainstream