replace let* with let when possible

Using let instead let* when possible is a good coding style, at least in
Common Lisp. Actually, let may have better performance since let can do
parallel assignment, while let* can only do sequential assignment.
Merge pull request #46 from xiaohanyu/devel

replace let* with let when possible
remove op/theme-directory from op-vars.el

Less is more, if a default options is good enough, then there's no need
to make it configurable. Second, the html template and theme files are
all static files, but there are two different ways in org-page to handle
html template and theme files, consistency should be keeped.
Merge pull request #49 from xiaohanyu/refactoring/remove-op-theme-var

remove op/theme-directory from op-vars.el