Fix triggering blur error manually and other errors

[Alex1990]

• When you trigger blur event to validate data before sent to the server, it will lead to error. See demo: http://alex1990.github.io/misc/placeholder-polyfill-test/trigger-blur-error-demo.html. (Do not input anything, just submit the form).
• Changing the type of input with setAttribute doesn't throw error in IE7/8 simulated by IE9+. But, input.type = someType will throw error. This can fix the password input dot problem in IE7/8 simulated by IE9+.
• Fix the set hook chainability and clearPlaceholder error. You can check the code for details.

The demo after fixing the above errors: http://alex1990.github.io/misc/placeholder-polyfill-test/placeholder-polyfill-test.html

[amerikan]

@Alex1990 looks good. I'm going to do some additional testing on it. Can you rebase? So that I can merge. Thanks!

[Alex1990]

Okay! @amerikan I've fixed the chainability of the val() method for password. For example:

  $('#password').val('aaa');
  $('#password').val('');

  $('#password').val('aaa').val('').val();
  $('#password').val();

  $('#password').val('aaa').val('bb').val();
  $('#password').val();

  $('#password').val('').val('').val();
  $('#password').val();

You can check the online demo http://alex1990.github.io/misc/placeholder-polyfill-test/placeholder-polyfill-test.html.