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.

Thanks to Adriaan Vuik for reporting.

Thanks to Bernard van Heck for reporting.

Thanks to Bas Nijholt for reporting this.

allow attaching a lead to a builder that contains sites that are not commensurate with the leads's symmetry

* Only extract the version from Git when the top level directory of the Git
  repository coincides with the root directory of the Kwant distribution

* Disregard tags on branches that were merged-in (with Git >= 1.8.4).