Add the ability to precompile assets to the source directory.

[robotdana]

This enables using safe mode and the default jekyll setup on github pages,
as long jekyll build is run once in unsafe mode after files in
the _assets directory are modified.

I'm doing this because jekyll-assets is the only plugin I'm using
and this gives me the flexibility to make changes to _posts using
the github interface or something.

[coveralls]

Coverage remained the same when pulling 415680d on dnl:master into 9ac861a on ixti:master.

[robotdana]

This change did require me to put this in my layout, as I wasn't sure how to tell jekyll-assets what files to write without using the liquid tags, and it doesn't allow me to use the cache busting.

{% capture assets %}
    {{ 'application.css' | asset_path }}
    {{ 'libs/modernizr.js' | asset_path }}
    {{ 'libs/jquery.js' | asset_path }}
    {{ 'application.js' | asset_path }}
{% endcapture %}

I have a plan for restoring cache-busting though. Writing an _include containing just the stylesheet and javascript links when run in unsafe mode, that can be included even in safe mode.

[ixti]

I don't see any value in this workflow. You can't have any 3rd party plugins on github pages anyway, so you won't be able to have your asset_path helper or tag running on GH. That leads to the question how else you would refer to dynamically compiled assets? If using "well-known" path, then there's no need to use jekyll-assets at all IMHO, it's easier to use sprockets CLI instead, so you will have same structure (_assets) and a small rake task that will execute sprockets to prebuild assets. Actually you can even use JekyllAssets interface to get to force compile assets:

site     = Jekyll::Site.new Jekyll.configuration Hash.new
manifest = Sprockets::Manifest.new site.assets, "_site/assets/manifest.json"
manifest.compile "application.css", "application.js" # ...