fixes #36

this fixes #35

Running 2to3 removed a line in the graphene.py tutorial script, which
broke the diff file. We manually update the diff file to fix this.

It was not previously made explicit anywhere in the docs on how
the leads were numbered. This commit adds a line to the docstring
of `kwant.builder.Builder.attach_lead` which explains the meaning
of the return value. It also adds a few lines to the first tutorial
explaining the numbering.

This undoes 00b0536c, reaching the aim of clearer error messages in a more
systematic way.