New functionality to share with Facebook and Twitter selectively

Introduced the new method called "shareA" that allows sharing with Facebook or Twitter selectively (using SLComposeViewController). "shareA" takes four parameters:

1.  Service Name (Facebook, Twitter or Sinaweibo). -- string
2.  Text to tweet -- string.
3.  URL to post -- string.
4.  Path to the image to attach -- string.

A call would look something like this:

<code>
window.plugin.social.shareA('Facebook', 'I love guitar!', 'http://www.victortsaran.net', 'my image.gif');
</code>

I am willing to see this merged into the main "share" method but I didn't want to do anything major until I hear back from the original author.

Looking forward to your feedback.
Victor