Use configure :build to prevent weird errors
Added documentation about block for import_assets
Fixed documentation for the new api and add a hint
Merge branch 'master' of https://github.com/middleman/middleman-guides into feature/addition_asset_pipeline

Conflicts:
	source/basics/asset-pipeline.markdown
Merge pull request #328 from dg-vrnetze/feature/addition_asset_pipeline

Add documentation for new import_asset-implementation