Added possibility to get receipients and their email addresses

[MGlike]

I tested the possibility to actually receive information about possible receipients prior to sharing files with them. You cannot only get full names of the users, but also their email address (which is what owncloud needs to be able to share to that specific person).

From this moment on I know about 2 problems in this code:

1. The parameter item_type is needed to do this HTTP request, but i don't know why. Still included it in the function. It is also stated as required in the owncloud documentation
2. _parse_xml_for_wanted_element is not really abstract, might need some tweaking to be more multifunctional.

I hope this helps. Thanks

Also:
There seems to be a problem with the owncloud documentation. The examples below of Receipient API in https://doc.owncloud.org/server/10.0/developer_manual/core/ocs-recipient-api.html are just taken from https://doc.owncloud.org/server/10.0/developer_manual/core/ocs-share-api.html (section "Delete A Share"). Maybe someone of you is working on that and might want to have a look on that.