Add file transfer functionnality

[NeomMob]

Not sure if it can be done using partial XMMP implementations of Facebook / GTalk but would be nice to be able to transfer file between 2 ospreyapp

[binarybucks]

It is not a top priority as I'd have to implement the non-standard GTalk filetransfer XMPP protocol extension first. When the mayor bugs are ironed out in the future I might add the functionality though.