Merge branch 'release/0.5.1' into develop
Move test helpers out of asset test case
Test static asset source property
Test that assets are iterable
Remove unused __str__ from BaseAsset

Because Asset class has its own __str__ method, and StaticAsset has no
unicode representation.
Test building an asset
Add compat module for Python 3 support stuff
Test that assets are convertible to bytes
Avoid code duplication in tests
Fix cssmin and slimit compressors
Fix bytes from compat to handle iterables properly
Add hexdigest property for static assets
Add hexdigest path property for assets
Store public assets info in the manifest file
Better mock.patch usage
Fix syntax error
Fix Python 3 support in manifest tests
Hide manifest file
Fix manifest creation for empty assets dir
Add post-processor to fingerprint paths inside CSS files