Add LinkType to differenciate different link types

[oberien]

Fix #141

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[oberien]

I signed it!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[raphlinus]

This looks good to me. Of course it is an API breaking change, but I think it's reasonable to unfreeze the API.

I'm still locked out of merge permissions but expect to resolve that soon.

[raphlinus]

Sorry about the merge conflict. I'm going to go ahead and do an 0.2.0 now without it (partly to minimize changes for clients), but there'll be scope to get this in soon.

[oberien]

Ok. Should I resolve the merge conflicts now or are you planning on adding more code that could result in further conflicts?

[raphlinus]

Hmm, let's try to get stuff in now. Not sure what the release strategy will be but we can figure that out later.

[oberien]

I rebased the linktype branch on the current master.