Support UTF-8 in Markdown files

Liquid supports Unicode strings, but reading the file off disk is defaulting to US-ASCII, which means Liquid barfs on legitimate UTF-8 characters (e.g £, €). An example of this behaviour can be found in my comments on mojombo/jekyll#836 .

This change forces it to assume UTF-8 for read files, which will be safe for US-ASCII as it's entirely contained by UTF-8.

About 1/4 of the changes lines are just updates to existing tests to take the changed post numbers/categories into account.