Enhancement/attributes

Hi @danslo,

This time I worked to add a way to import attributes, attribute sets, attribute groups and to associate attributes to attribute groups in one or more attribute sets you specify.

I added three API methods : importAttributes, importAttributeSets and importAttributeAssociations.

I added a new behavior : delete_if_not_exist. Works with attributes, attribute sets and attribute associations import. Any data you do not specify in your array will be deleted! Send your array, data that there are in Magento and not in your array are deleted.

You are free to merge PR #17 which is the base of this one, and then merge this one.