User registration

- [x] Implemented RegistrationController
- [x] Created registration template 
- [x] Implemented input data validation on front-end
- [x] Created Sinatra routing required for registration
- [x] Added OPTIONS method for Sinatra required for CORS POST requests

```javascript
.factory('RegistrationService', [... function(){ ...
...
var submitRegistration = function(){
    $scope.regform.submitted = false;
    if ($scope.regform.$valid && this.captcha == this.enteredCaptcha) {
      $http.post("http://localhost:9292/register", this.user)
        .success(function(data){
          $location.path("/");
        })
        .error(function(response, status, headers, config){
          if (!!response.username && response.username.indexOf('has already been taken') !== -1){
            reg.alreadyTakenUsernames.push(reg.user.username);
            reg.checkUsernameUniqueness();
          } 
        });

return {
    submitRegistration : submitRegistration 
}
```

in controller:

``` javascript
this.submitRegistration = RegistrationService;
```