Update wc-invoice.php

Added functions to: (i) Check if WooCommerce is active; (ii) Detecting WC version; and (iii) Maintain compatibility with 1.6.6 and 2.0
Update wc-invoice.php

Cleaned up Detecting WC version & Maintain Compatibility with 1.6.6 and 2.0 sections; RE: Check if WooCommerce is active, moved close '}' to the last line of code; updated syntax to unset 'session.'
Merge pull request #4 from vDevices/Update-for-WooCommerce-2.0

Update for woo commerce 2.0