Add create_item bulk methonds to overload set

[bkircher]

Currently, you can only create one item at a time, e.g. a message

create_item(const message& the_message, ews::message_disposition disposition)

I think it is absolutely possible to create a bunch of items at once, see here.

Add

create_item(const std::vector<T>& items, ...);

whereas T is one of ews::message, ews::calendar_item and so on to overload set. Take special care of the response handling when the creation of a single item fails.

[bkircher]

Done with dcaf38c