#71 Allow labels on all chain edges

[sampablokuper]

At the moment, adding labels to edges that branch or join chains in TikZ seems to be unsupported. See, for instance the discussion (and links) here: http://tex.stackexchange.com/q/40848/15911

The available workarounds are cumbersome and unreliable.

Please add to TikZ support for labelling any edge in any chain.

[Till Tantau]

The chains library is now outdated. The graphs library and its friends are more powerful and easier to use. There, adding a label to edges is very easy. I do not indent to work on the chains library any more.