Integrate payment processor

__Major parts of this pull request__

* payment model added to donate to Paypal.
* payments controller added to get input from user and send donation information to payment model for processing.
 form added in welcome/index.html page to show form to user to give payment amount, first name and last name as   input.
* donate path added in routes


__Todo__
* rake  db:migrate

 Please check the image to see what will happen after donation.
![donation](https://f.cloud.github.com/assets/530250/1579712/d8685228-51a4-11e3-93ea-cda009475a7e.png)