make plaintext paste work in IE

here's one approach to resolving #340.

here's stuff I found:
1. the `event` object (`e` in the code) doesn't have a `clipboardData` object in IE. instead, `clipboardData` is on window.
2. once we finally get `clipboardData`, its `getData` method [takes different strings for arguments in IE](http://msdn.microsoft.com/en-us/library/ie/ms536436(v=vs.85).aspx). Looks like we want `'Text'` here.
3. IE11 no longer supports `document.execCommand('insertHTML' ... )`, so we need to use a different strategy for inserting the content into the document.