Fixed issue in uninstall script

When using 'uninstall.php' the plugin should always check for the WP_UNINSTALL_PLUGIN constant, before executing. The WP_UNINSTALL_PLUGIN constant is defined by WordPress at runtime during a plugin uninstall, it will not be present if 'uninstall.php' is requested directly.
Merge pull request #10 from IgorCode/patch-1

Fixed issue in uninstall script