Implement dynamic text-replacement macros

A macro, in wiki markup terms, is a piece of markup which does a string
replacement at page render time with a dynamically generated piece of HTML.

This patch also includes an example implementation of a very simple "list
all pages in the wiki" macro, called AllPages.
Merge pull request #51 from anchor/macros

Implement dynamic text-replacement macros
Implemented abstraction layer for Grit.
Merge pull request #92 from repotag/git_abstraction

Implemented abstraction layer for Grit
Improve wiki subclassing. Closes #36