Eliminate _MODULE_DIR variable

It is only used in one place, where it can easily be replaced by its
expansion. The reason for removing it is that it depends on the name
of the project, and I intend to read this using a routine it the very
directory it is including, hence the need for a different approach.
Add description back to dune.module

We want to read this from a declarative file instead of having it
in the build system source.
Read project properties from dune.module

We cannot generate dune.module because that file is read by dunecontrol
before the build starts. It therefore make sense to let it be the
original and let the build system read it instead of duplicating the
information in CMakeLists.txt

This module is a adaption of Andreas Lauser's OpmParseDuneModule
in opm-material, modified to handle the more sparse info provided by
the other modules as well.
Added note about where to make likely modifications

People will head for CMakeLists.txt when making modifications (such
as adding a new file). This note directs them to where they should
rather look.
Merge pull request #287 from rolk/286_info

Read project settings from dune.module