mw.loader: Fixes for <script> and <link> loading.

mw.loader: Fixes for <script> and <link> loading.

addScript:
* Integrate fixes from jQuery where this function is based on.
  * Set properties instead of attributes (faster, shorter and
    less potential bugs)
  * Set async property (why not!)
  * Remove redundant try/catch. Whatever this was for,
    apparently not needed anymore. Follows-up 41d636d3.

mw.loader.load( , text/css ):
* (bug 41331) Security warnings in IE7 for protocol-relative
  urls for stylesheets in mw.loader.load.
  Fixed by using properties instead of attributes. Not sure why.
  Probably related to how IE "normalises" urls when set as
  attribute, which would not happen as much when setting a
  property.

importStylesheetURI:
* Remove redundant .type property, fix space.

Change-Id: I04f1775213633e6822b4f982fd43a35b19d19531