Docs: use the base layout when mako_layout=='epub'

  - https://readthedocs.org/projects/sqlalchemy/downloads/epub/latest/ renders with the full template which is unreadable on e-readers
  - in the makefile the template-variable mako_layout is set for target: epub which makes mako use only the base layout.
  - this is the more elegant solution as proposed to #133
Merge pull request #133 from hiaselhans/doc_epub

Docs: use layout.mako only when mako_layout=='html'