Merge pull request #182 from stbuehler/forget_dependencies

Call forget_dependencies_if_outdated in run instead of load
Remove .gemtest from files in gem
Add gem build testcase
Update release notes for 3.4.3
Update version to 3.4.3
Set release date for 3.4.3 to today
add compress option to less filter
Add asset registry

The asset registry keeps track of extra files created during compiling item
representations, so they are not pruned by nanoc.

For example a latex helper could generate pngs from inline latex.
asset helper to provide a nice wrapper for the asset registry

The asset helper helps finding unique filenames to store assets as, and
registers them in the asset registry for the current item representation.
cache helper to cache inefficient transformations on disk

Adding a new article can lead to a total recompilation of all articles
(menues, tag clouds, ...).
But the article content usually doesn't change, so running stuff
like pygmentize again is not needed.
Add asset registry to show-data