Modify EventLogSelectionTask to accept multiple values for source parameter.

Values in config files are specified on separate lines.  E.g.:

  [event-logs]
  source = s3://fake/input/
           s3://fake/input2/
Update get_filename_safe_course_id to be more restrictive.

Instead of hardcoding knowledge about colons, just replace all
but a well-defined set of supported characters.

And fix a PEP8 violation.
Merge pull request #17 from edx/brian/multiple-event-src

Modify EventLogSelectionTask to accept multiple values for source parameter.
Merge pull request #23 from edx/brian/fix-safe-file

Update get_filename_safe_course_id to be more restrictive.
Fix acceptance tests given latest changes to exporter