fix(checkbox): rename tabindex to tabIndex (#2953)

* Renames the checkbox `tabindex` input to the `tabIndex` input. This is consistent with other API's like for the slide-toggle and also with native inputs.

src/lib/checkbox/checkbox.html

@@ -7,7 +7,7 @@
            [checked]="checked"
            [disabled]="disabled"
            [name]="name"
-           [tabIndex]="tabindex"
+           [tabIndex]="tabIndex"
            [indeterminate]="indeterminate"
            [attr.aria-label]="ariaLabel"
            [attr.aria-labelledby]="ariaLabelledby"

src/lib/checkbox/checkbox.spec.ts

@@ -727,7 +727,7 @@ class MultipleCheckboxes { }
 @Component({
   template: `
     <md-checkbox
-        [tabindex]="customTabIndex"
+        [tabIndex]="customTabIndex"
         [disabled]="isDisabled"
         [disableRipple]="disableRipple">
     </md-checkbox>`,

src/lib/checkbox/checkbox.ts

@@ -139,8 +139,8 @@ export class MdCheckbox implements ControlValueAccessor {
   get disabled(): boolean { return this._disabled; }
   set disabled(value) { this._disabled = coerceBooleanProperty(value); }
 
-  /** @docs-private */
-  @Input() tabindex: number = 0;
+  /** Tabindex value that is passed to the underlying input element. */
+  @Input() tabIndex: number = 0;
 
   /** Name value will be applied to the input element if present */
   @Input() name: string = null;