The total density of states now integrates to `N`, the size of the system.

In addition to basic updates to skip permanent redirect, remove an
obsolete reference to macports installation instructions on the main
website.

  `integrate` is a more accurate name for a function that integrates
   the spectral density. Docstrings and tutorial updated.

closes #125

The site() method was never widely advertised, so no one will miss it.

This was deprecated several releases ago

Current implementation uses ``kwant.physics.Bands``. This example fails
when one substitute ``k_x**2`` with for example ``k_x**4`` due to farther
then nearest neighbours hoppings. Using wraparound provides also easier way
to extend it to multidimensional band structures (except of plotting).

The Mathjax CDN is shutting down:
https://www.mathjax.org/cdn-shutting-down/

Closes #116.

Closes #113.