Fixed namespaces for unit tests.
Fixes to the page linker. Added unit tests.
Fixed a null pointer bug.
Added ability to create posts for other blogs in `MockFileSystem`.
When a tag or category listing page is not available for a given blog,
look for the default one (no blog key) in the theme or resource folders.
Slightly more readable code.
Removed unused method.
Introduced the `ExecutionContext`:
- Keeps track of what is considered the "current page", which is set during both
  content and layout rendering.
- Make the `pcurl` family of Twig functions use that to figure out what default
  blog to use, through the `getDefaultBlogKey` helper function.
- Removed useless stats footer in debug mode (you can get the same info with the
  debug window).
- Added lots of fucked up tests.
Merged changes.
If the cache is not valid for a bake, only re-bake content, not assets.
Correctly handle slugified tags when previewing a website:
- Slugify tags when used in filter clauses to match posts correctly.
- Added some unit tests.
Fixed some unit tests on Windows.
Fixed baking of tags/categories with spaces in them.
Fix the `tag` and `category` template variables when they've been "slugified".
Fixed URL parsing on Windows.
Handle missing $server['HTTP_ACCEPT_ENCODING']
Added `site.pages` template data to list all pages recursively.
Added unit tests.
Updated CHANGELOG.
Skip page assets directories in the linker.
Added support for filtering/limiting the `site.pages` iterator.
TODO: Try an unify things with the `PaginationIterator`.