First step to batch processing of JS files

Make the external parser capable of processing multiple files. Adapt
the existing codebase to read the new result format. Don't actually
pass multiple files to the external tool yet.
Process JavaScript files in batches

JS files are now processed in bulk. Other files are still processed
individually (but filebatch.rb contains stubs to allow other
files to be processed in bulk too - see javascript_source_batch.rb
for inspiration).
Sometimes JavaScript files just can't be parsed

Recover from a failure in parsing a JS file the second time (it's
already failed to be parsed as part of a batch)
JS parser only notify about batch failures when needed

i.e. when actually processing a batch of files. Don't print a warning
message if we're only processing a single file.
Code cleanup in individual JavaScript file processing

Use :symbolize_names in JSON.parse() to do some of the work
Rename filebatch.rb to file_batch.rb
Don't fire up external JS parser for zero files