support for subclasses + switch to yaml frontmatter

[jonstoler]

• add subclasses to pages with subclass page metadata
• use full package names for subclasses (eg net.flashpunk.Entity rather than Entity)

See Entity.md for an example. I haven't filled in other classes yet.

[jonstoler]

• reformatted how pages are rendered to use yaml frontmatter instead

Note: this means we'll need to reformat every page, since class descriptions are no longer part of page contents. Example code can stay where it is.

[zachwlewis]

Do we need to specify subclasses? I think if each class marks it's parent, we can walk backward.

parent: net.flashpunk.Entity

[jonstoler]

I think we're actually fighting for the same thing here, but with different terminology.

To be clear: the subclass tag specifies what this particular page is a subclass of. I realize now this is kinda confusing.

For instance, Image would have net.flashpunk.Graphic under its subclass tag.

Changing to parent or extends or superclass sounds like a good idea to me. I chose subclass because the HTML outputs as "subclass of ____", but I can see how this is confusing. I'll change it.