refactor: changes in html

 - Let "options" be named "state".
small changes in comments
refactor: minor changes in markdown's public API

 - the `callbacks` argument is now named `renderer`
refactor: changes in markdown

 - the `cb` member in `hoedown_markdown` is now called `rndr`.
 - the `rndr_*` local utilities are now unprefixed.
refactor: changes in markdown

 - stick to `md` for `hoedown_markdown` variable names
refactor: changes in html

 - the struct `hoedown_html_renderopt` was renamed to `rndr_state`
examples: hoedown_markdown was typedef'ed for some reason!
Merge pull request #31 from jmendeth/the-big-refactor

The Big Refactor