Fix issue where donation_exists() throws an error due to NULL value

Fix issue where donation_exists() throws an error due to NULL value when viewing donations in admin area.

Added Widget For Donation Form
Added Seperate function to get Donation Projects donation_projects()
Added 2 Actions wc_quick_donation_before_form &
wc_quick_donation_after_form for donation form.
Minor Bug Fix.

* Plugin Activation Issue Fixed.

* Configurable Min & Max Donation Amount
* Custom Error Messages
* Separate Menu with donation order listings
* Fixed Order Notes And Order Meta Added For All Products
* Fixed Saving Donation Order Id In DB [Before It Stored All Order IDS]
* Minor performance fixes
* Code Clean Up
* Removed Row Action [Quick Edit , Trash & Duplicate] Options For
Donation Product In Product Listing

* Internal Server Error / php error fixed while adding donation to cart
[WP : 4.1 | WC : 2.3.3]
* Minor Bug Fix

Plugin Activation Issue Fixed.

Redirect User After Donation Added To Cart [Cart Page / Checkout Page]
Select Your Preferred Payment Gateway For Donation
Custom Email Template For Donation Processing
Custom Email Template For Donation Completed
Some Minor Bug Fix

* Base Version