Change \A and \z to ^ and $ in regex

Hey

I tried to add a email validator to my application, based on the [rails guides on validations](http://edgeguides.rubyonrails.org/active_record_validations.html#custom-validators), like this:
```ruby
validates :email, format: { with: /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\z/i }
```

However javascript doesn't seem to support `\A` (start of string) and `\z` (end of string), but only `^` (start of line) and `$` (end of line). However i can't use that in my validation, because rails will throw a error telling me to use the multiline options, which is not really what i want. 

I therefore made a small fix for this that replaces `\A` with `^` and `\z` with `$`. This would properly be temporary if you still plan on implementing a XRegExp plugin, but it was the one most important differences between regex in ruby and javascript I could find :)