Added method 'available' back in.

The original version had a method, 'available', which was very handy for checking upfront if social was available, thus allowing alternatives to be put into place earlier. I see that your current version has it implemented as a failure inside of 'share' itself, but thats not very usable for pre-determination of support.

The JS file still has the 'available' method in place, so i just added the old method back in using the new Cordova 3.x plugin methods.