Split and move xfer package.

[tomwilkie]

Fixes #641

[paulbellamy]

Not a fan of:

import (
  "github.com/weaveworks/weave/common"
  xcommon "github.com/weaveworks/scope/common/xfer" // sometimes called just `common`
  "github.com/weaveworks/scope/probe/xfer"
)

Maybe:
probeXfer "github.com/weaveworks/scope/probe/xfer", or
commonXfer "github.com/weaveworks/scope/common/xfer"

In general common "github.com/weaveworks/scope/common/xfer" seems a bit confusing, since common is already the name of another package.

[tomwilkie]

@paulbellamy I agree (x)common is pretty ugly; but the cAmElCaSe seems pretty ugly too. How about we rename probe/xfer to be proble/appclient?

[paulbellamy]

probe/xfer -> app/client ?

Edit: or probe/appclient, either one.

[tomwilkie]

@paulbellamy PTAL