Add load_path to the YAML loader

The YAMLLoader can now take in a list of directories for load_path.
It uses standard YAML syntax for list items.

load_path:
 - dir1
 - dir2
 - dir3

Or it can take in a single item that it will append to the load_path
array.

load_path: dir1
env.resolver.resolve_source_to_url (bundle.py:644) requires a full_path
Test cssrewrite not invalidating data uris.
Allow nested bundles to be empty.

It turned out that in certain open-ended configuration scenarios, such a thing
can be very useful, and an exception in those cases a headache to workaround.

The bundle structure as a whole still mustn't be empty.
Implement JST/Handlebars as concat() filters.

This is a much better approach, no longer requires abusing multiple filter
functions to work in concert, fixes the caching issue of #188, and in general
should save us some headache.

As part of this, FilterTool.apply_func was changed to raise an exception if no
filters run. This allows us to differentiate between no filter running and a
filter not returning anything. We didn't end up needing this here in the end,
but it is useful nonetheless.
Fix cssrewrite filter destroying data: uris.

Other non http/https protocol links were also affected. Closes #184.
Improve PyScss filter docstring.
Add config options for root/url to PyScss filter.

This is not only generally useful, but critical for environments like
Flask-Assets, where no single default directory/url pair exists.
Allow query_url_mapping() without url/directory.

This allows Resolver.query_url_mapping() to be used even in installations
where no fixed url/directory is available.
Document the "depends" keyword of bundles
Come up with a list of changes for 0.8 release.
Add instructions for releasing a new version.
Updated AUTHORS file.

Generated using:

    git log --pretty=format:"%an <%ae>" | sort -u

With manual duplicate removal.
Update version number of production release.
Set version number in Upgrading section.
Set version number to 0.9 dev.
Update release instructions.
Do not run cssutils filter in debug mode.

This was added in 72122af96f9456a9dca0ddbb3adaea2306454a38 without further
comment, but it doesn't make sense to me anymore. It's a compressor, it
should be production-only.
Added json:{path} to the list of valid manifest values.
Fix logic in css compilers doc