php notices and method slug

Fixes php notices (WP_DEBUG):
Notice: Undefined offset: 1 in
/srv/www/wp-parser/public/wp-content/plugins/wp-parser/lib/class-plugin.
php on line 172

Notice: Undefined variable: post_data in
/srv/www/wp-parser/public/wp-content/plugins/wp-parser/lib/class-importe
r.php on line 326

Replaces double colons in method slugs with a dash before
sanitize_title() is used.
Merge pull request #107 from keesiemeijer/import-item

php notices and method slug