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

This option is not necessary since a similar effect can be achieved by
filling an emtpy builder and using Builder.update.  Furthermore,
removing it opens the door for making fill "sticky" in a consistent way.

Our baseline is Debian stable (Jessie) where possible.  The only
dependency that cannot be satisfied is sympy; Debian stable does not
have a Python 3 package for this.  We set the required sympy version to
that present in Ubuntu 16.04 LTS.

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.

If "expr" is already a SymPy object and "locals" are provided the
RuntimeWarning will be triggered.

closes #125

The default relwidth was 0.03.  This means 33 "widths" per longest
plot dimension, and 67 stream lines (we show 2 streamlines per
width by default).

67 streamlines are too much for small square plots.  Increasing
relwidth to 0.05 produces nicer looking results and the current
plotter also runs much faster!