Fix for selectionStart == 0

Hi,
This is a fix related to @schulzch fix about selectionStart == 0. Also I added two tests when doing insertAtCaret into an empty textarea.

Before fixing this issue I  tested on OSX 10.9 Chrome 31, FIrefox 25 and Safari 7.0, as you can see in the image tests were failing:
![with errors](https://photos-2.dropbox.com/t/0/AABaFdm1bXVv6Z8Fr0hUHd43QYUy6CRNVuYE9i6appAAHg/12/320558/png/1024x768/3/1386986400/0/2/jquery.caret_test_errors.png/2N8ZV6iWNgJnzW9vB6v1G3w0awBN1rFmWVwFIO_uGLw)

After the fix:

![no errors](https://photos-5.dropbox.com/t/0/AACUpxA8hZDgbah6-013VADL2XQSxyNn_Ys9eC0Ay-fAnQ/12/320558/png/1024x768/3/1386986400/0/2/jquery.caret_test_no_errors.png/NjyanyrxtuuAFWu0fdsTFOqQNqWU-kR7xzyhZ2Vq_1U)


Thanks for your nice plugin :)