Add plugin to associate simple products with bundle products.

fixes #21

To associate products to options in a bundle you need at least a column bundle_skus with the option-title and product sku separated by a colon. More optional options can be set. The defaults for these options can be set in the configuration. More products can be separated by semicolon.

The format for bundle_skus is:
"Option title in admin store:sku-of-product:[selection_qty]:[selection_can_change_qty]:[position]:[is_default]:[selection_price_value]:[selection_price_type];Option title in admin store:sku-of-other-product"

You may control the options with the bundle_options column. You can delete options by prefixing the title with a minus "-". If you want to delete all options you have to specify an option with a minus followed by an asterisk "-*". More options can be separated by semicolon.

The format for bundle_options is:
"Option title in admin store:[Option title in item store]:[type]:[required]:[position];-*"