Check if email exists

example:
```js
          function() {
            actions.user.emailExists(
            {
              silent: true,
              data: {
                email: newValue.value
              }
            }, function(err, result) {
              // treat this response only if its the last we made
              if(curReqId === self.state.email.reqId) {
                var newState = err || !result ? "invalid" : "valid";
                newValue.state = newState;
                self.setState({
                  email: newValue
                });
              }
            });
```