doc: use better fonts, minor layout changes

halmd.css_t: add missing style "p.rubric"
doc: minor changes
doc/readme: mention Nicolas Höft explicitly
doc: include develop.rst in TOC tree

Let the developer guide start with this overview.

Rename directory developer/ to develop/
doc: revise section on H5MD output files
doc: update section "Getting started"
utility/realname: do not return an empty string

When the variable 'gecos' contained the string ",,,", it returned an empty name,
which causes hdf5 to fail writing the attribute.
doc: set content width to 72em
doc: add cuda package to ubuntu guide
packages.mk: update HDF5 to 1.8.12