Updating e-mail validation for signup view in examples

This is a minor thing - when validating the e-mail within `examples/client/views/signup.html`, it accepts e-mails with the form shown below: 

![before](https://cloud.githubusercontent.com/assets/1226900/4054315/5568a7cc-2d8d-11e4-8734-02e9f0ea7cee.png)

By using `ng-pattern` to further validate the e-mail, it produces this result:  

![screen shot 2014-08-26 at 10 04 40 pm](https://cloud.githubusercontent.com/assets/1226900/4054369/94e9acc4-2d8e-11e4-8e57-04bdb3087fe7.png)

The form will display an error message if the e-mail is not of the form `alphanumeric@<domain>.com`. This regex is not full-proof, but it will at least prevent e-mails of the form `alphanumeric@alphanumeric` from passing.