Updated html export to org v8 standards

[jonEbird]

Replaced use of org-export-string with org-export-string-as to match
updated org coding standards.

This specifically addresses issue #391

[djcb]

Looks good, but how does this affect users that do not have org 8.x yet?

[jonEbird]

All my machines are up to date with org version 8.x, so it's harder for me to test an older version of org. I did look at the git log for org-mode and see that the new function I'm using was introduced with version 8 (commit 8dd2bfc2) Perhaps I can rework the patch to conditional select which method to use with a preference on the newer function if available?

[djcb]

What want to avoid is that the org-code suddenly starts breaking for some users using older org versions... Probably the best would be to have some mu4e-org-old.el and mu4e-org-new.el (with better names!) and then have mu4e-org.el automatically load the one based on the org-version.

[jonEbird]

I suppose that would help keep the code simpler in each file but I also wonder if the only conditional place needed is in the org~mu4e-mime-convert-to-html function.

What about keeping the current file named as it already is in "org-mu4e.el" and have the newer one called "org-mu4e-ox.el" or "ox-mu4e.el"? The updated exporting system in v8 of org-mode introduced lisp/ox.el as the core, generic org exporter (ox), so it would imply that it's using the updated exporter.

[jschwab]

A question relevant to this and other issues is "what versions of emacs does mu/mu4e officially support?". Is this written anywhere?

[djcb]

Merged -- thanks! I have split off a 'legacy' version into org-old-mu4e.el, so we can depend on org 8.x org-mu4e.el.