feat(strict mode): turn on ECMAScript 5 strict mode

- add 'use strict'; statement to the prefix file
- configure closure compiler to use the ES5 strict mode
- strip all file-specific strict mode flags after concatination

Closes #223

src/angular.prefix

@@ -21,4 +21,5 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
+'USE STRICT';
 (function(window, document, undefined){