Add development mode

In development mode, we do not bundle assets, but copy them as is to
the destination directory.
Rename BundleMarkup to AssetTagMarkup

It's used not only for bundled assets, but for development assets as
well.
Update files in gemspec
Prepare test site by copying fixture files, and explicitly tell which test site to get
Add test for ministamp when asset source changes
Prefer Hash literal for shorter syntax
Extract getting asset's path from generated index content file
Add tests for minibundle in production mode when assets change
Rename minibundle's integration test source to include used mode
DRY: We want the first element always
Add integration tests for minibundle's development mode
Share fixture dir and source config in one place
Merge branch 'add_dev_mode'