RIS import creates unlinked webpage items outside of parent

[dstillman]

Should create child attachments instead.

Note that items are not placed in Imported folder.

Reported here: ​http://forums.zotero.org/discussion/1279/

[dstillman]

(In [1752]) Fixes #759, RIS import creates unlinked webpage items outside of parent

Also:

• Made linkFromURL() always return the attachment itemID, performing a HEAD request asynchronously to get the MIME type if necessary
• Removed test for linkFromURL() return value in Translate::_itemDone(), which was incorrect anyway
• Removed a couple unused return assignments from linkFromURL() in _itemDone()