gen_gallery.py: new generate_gallery_html(), new --link-prefix option
gen_gallery.py: new generate_gallery_html(), new --link-prefix option

- update generate_rst_files() docstring
- gallery page works

sfepy/sfepy,BubuLK/sfepy,BubuLK/sfepy,lokik/sfepy,RexFuzzle/sfepy,rc/sfepy,vlukes/sfepy,sfepy/sfepy,lokik/sfepy,RexFuzzle/sfepy,lokik/sfepy,sfepy/sfepy,rc/sfepy,RexFuzzle/sfepy,vlukes/sfepy,RexFuzzle/sfepy,rc/sfepy,BubuLK/sfepy,lokik/sfepy,vlukes/sfepy