feat(ngRemove): directive to remove a portion of the DOM tree (similar to ngHide)

I added a directive called `ngRemove` that I think could be useful. This works similar to `ngHide` except instead of setting the `display: none` css it will remove the element from the DOM.

There is already a comment in the code about refactoring `ngHide` *"//TODO(misko): refactor to remove element from the DOM"*. It seems to me `ngShow/ngHide` imply setting visibility rather than removing an element.

##### Some example cases when `ngRemove` is helpful:

When using bootstrap in the situation shown in the HTML snippet below the bootstrap css uses selectors like `.btn-group > .btn:first-child` and `.btn-group > .btn:last-child` to properly format a series of buttons.  In this example if you just hide the last button `#btnCategories` using `display: none` the previous button `#btnUpload` won't be styled correctly -- the `#btnCategories` button needs to be removed from the DOM to trigger the `:last-child` selector on the `#btnUpload` button.

    <div class="btn-group">
      <button id="btnHome" class="btn btn-primary"><i class="icon-home icon-white"></i> Home</button>
      <button id="btnUpload" class="btn btn-primary"><i class="icon-upload icon-white"></i> Upload Document</button>
      <button id="btnCategories" class="btn btn-primary"><i class="icon-edit icon-white"></i> Manage Categories</button>
    </div>

When using alternate inputs in a form (depending on application settings) it seems cleaner to remove the unused alternates instead of hiding them (in some cases they may share the same `name` or `id` attributes).  As an example, the code below will use `radio` inputs if the application is set to only allow a single selection and `checkbox` inputs if multiple selections are allowed.

    <div id="answer-options" class="controls" ng-init="answerData={}">
      <label class="{{slideData.answers.selectone && 'radio' || 'checkbox'}}" ng-repeat="answerOption in slideData.answers.options">
        <input ng-remove="!slideData.answers.selectone" type="radio" ng-model="answerData.radio" value="{{$index}}" />
        <input ng-remove="slideData.answers.selectone" type="checkbox" ng-model="answerData[$index]" />
        {{answerOption.text}}
      </label>
    </div>
    <button type="button" ng-click="submitQuestion(answerData)" class="btn">{{slideData.submittext || "Submit"}}</button>