Add appropriate package headers
Merge pull request #1 from webframp/feature/package-headers

Add conventional package headers plus requires for org 8+ (ox-html)
Clean warnings.
Clear warnings.
org-reveal-root in URL form.
Set character encoding to utf-8
add <meta charset="utf-8"> to HTML heading section.
add a tip about disabling heading numbers.
Support force split slides.
fix the internal link bug.
Add support for more reveal.js slideshow settings: rolling links, keyboard on/off switch, overview on/off, and transition speed.
Add support for loading extra stylesheets to customize themes.
Merge pull request #5 from gcv/master

Hi, Constantine, Thanks for your suggestion and codes!
add preamble/postamble support.
Add REVEAL_HTML keyword for embedding raw HTML contents.
Add custom variable and option keyword for preambles in head part.
Support custom slide size and scaling constraints.
Prevent type conversion export crashes when REVEAL_MARGIN, REVEAL_MIN_SCALE, or REVEAL_MAX_SCALE are not set in the input file.
Merge pull request #10 from gcv/master

Thanks, Constantine, It is really a bug I didn't notice. Thanks again for your quick correction!
add PDF.css to support print to PDF.