Viewer examples

Hi all,

In response to a question on the mailing list ([Lineprofile, how to retrieve data?](https://groups.google.com/d/msg/scikit-image/_rpFRHVe3zE/8H_TWQsfAkkJ)), I have updated as many examples as possible to make use of the APIs we created for retrieving overlay data from the viewer (#810, #865, #875).

I hope this can make it into 0.10. If there's any review comments, I'll make it a top priority to address them.