Improve peer host name resolution (closes #360)

- Take a QString in argument to avoid converting it twice. The caller already
  has the IP as a QString.
- Fix bug where hostnames coming from the cache would not the displayed.
- Fix bug where the resolved hostnames would not be displayed if the IP we know
  does not match the first one in the QHostInfo::addresses() list. It may come
  later in the list or may be formatted differently (possible with IPv6).
- Avoid requesting again host names for IPs that were already requested recently
  but whose resolution did not return a useful hostname.
Updated Changelog
Really fix issue #405 and #536.

Conflicts:
	src/mainwindow.cpp
Update italian translation by bovirus.
Updated italian language of nsis installer by bovirus.
Merge pull request #585 from sledgehammer999/it_translationv3

Update italian translation by bovirus.
Fix unhandled exception when trying to create torrents from files/folder in disk root.
Update Italian translation binary
Update disk space label in add new torrent dialog. Fixes issue #282.
Updated italian translation of NSIS installer by bovirus.