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
Merge pull request #94 from jamieoliver/feature/improved-wiki-subclassing

Improve wiki subclassing.
Better html_equal tests (closes #86).
Merge pull request #95 from repotag/better_html_equal_tests

Better html_equal tests
Update dependencies

 * github-markup 1.3.0

Ref #93
Merge pull request #99 from Mogztter/issue-93

Update dependencies (relating to https://github.com/gollum/gollum-lib/issues/93)
GitHub Markup is now using asciidoctor
Merge pull request #100 from Mogztter/asciidoctor-gem

Update installation procedure for AsciiDoc
Improve tag filtering performance
Cache Page#formatted_data

This should yield a threefold increase in performance on Gollum.
Merge pull request #101 from ut7/improve_tag_filtering_performance

Improve tag filtering performance.
Adds test for wiki filenames with embedded colons. Replaces https://github.com/gollum/gollum-lib/pull/90 .
Merge pull request #105 from repotag/filenames_with_colons

Adds test for wiki filenames with embedded colons. Related to https://github.com/gollum/gollum-lib/issues/89 .
Transparently remove paragraph tags surrounding code blocks. Resolves #97
Merge pull request #106 from repotag/strip_paragraph_tags

Transparently remove paragraph tags around code blocks. Resolves #97
Fixed empty TOC in _Sidebar issue. Resolves https://github.com/gollum/gollum/issues/814 .