feat(select): changed md-select to have select all feature

[praskarnam]

@gmoothart @ErinCoughlan @jelbourn @ThomasBurleson , Please review.

The multi select component of angular material does not have the below

• Select All
• When there are more than 1 selected then it keeps adding all of them to the selected label hence growing the component in size. Missing a label saying XX +n more as selected.

This commit adds the above functionality to the select component.

The below code shows how a md select can be written the feature of check all

      <md-input-container style="width:200px;">
        <label>State</label>
        <md-select ng-model="userState1" multiple id="hello" select-all>
          <md-option ng-repeat="state in states" value="{{state.abbrev}}" select-all>
            {{state.abbrev}}
          </md-option>
        </md-select>
      </md-input-container>

If you notice there is a attribute select-all which is added to the md-select which would add the check all option. Further you can see that the same attribute is added to the md-option to just display the check box for all the options. Adding to the md-option is optional where are adding to the md-select is required. The output would be as below

[ErinCoughlan]

@praskarnam This will be possible with the new changes to select currently in progress.

Adding checkboxes to each item in the multiple select case is PR #7455, which was already merged to master.

Allowing custom selected text (like "AL [+49 more]") is PR #7721, which also allows custom text like "Select your state" when the select is closed.

Adding custom header information, like a searchbar or select-all/none functionality is Issue #7383, which is in progress. Material UX has not specified how "select all/none" should appear, so that adds more flexibility for devs to customize it to their apps, for instance by adding a "Check All" checkbox above the item content, like you did here.