use i2p HTTP proxy for torrent fetches

[majestrate]

refiling of #110

when starting up, users often will load a torrent index in their i2p enabled browser choose a link and add it to XD which will fail because XD uses its own set of tunnels.

we should support using the http proxy with i2p for HTTP requests for fetching metainfo when adding a torrent.

[p060477]

...for users unlucky as me pls take a ride here:
#110
...to be able to understand...