Better Markdown support, Mac target, added options to API

* Updated to latest Discount, which includes direct support for Github-flavored Markdown.
* Added a Mac OS target.
* Extended the GHMarkdownParser API to allow for more customization of Discount options (such as auto-linking URLs and disabling smart-quotes.)
* Fixed a typo in the README and added instructions for Mac and using the new options.

NOTE: The submodule reference is to a Discount commit that won't exist until you merge in my earlier pull request that updates Discount.