Add automatic index creation during doc build.

Html or pdf specific index file created in makefile. Two rules were
added that copy correct index_*.txt file into build directory. The
index.rst file contains directive to include the file from
_build. Specifying _build/index in conf.py does not work.
Add generation of html documentation for developers.

Added new makefile rule devhtml that generates help in devhtml dir and
contains api documentation extracted by apidoc. Change html theme from
basic to default.