Allow fetch.txt with file URLs to validate

[avivace]

It looks like it passes all the tests (python setup.py test doesn't return any error) and it also succeds to validate the bag I added in LibraryOfCongress/bagit-conformance-suite#14 with this kind of entries in the fetch.txt:

file:///home/avivace/cern/example_bags/source/sachiel.png 42000 sachiel.png

It should effectively fix #153.

The fetch.txt validation now passes when:

• The URL scheme resolves to "file" after parsing (no value need for netloc in this case)
• The schema resolves to anything AND the netloc has a value too

if not (all((parsed_url.scheme, parsed_url.netloc)) or parsed_url.scheme == "file"):