Reorder imports

Put stdlib imports first, all together in one block, dividing them into
"import x" and "from x import y" and sorting them by module name.
Leftovers from ProxyService

I missed to update a comment.
Small LogService fix

Not really needed, it's just to make code more "clean".
Remove unused arguments from compute_changes_for_dataset
Store name on Service for easy access
Small improvements to sample configuration

Use three blank lines between sections, and don't wrap text when not
necessary.
Small fix to unit tests

Run only *.py files, excluding (for example) backup files (i.e. *~).
Make LICENSE.txt more compliant

Put whole notices for all licenses we use at the end of the file.

(Use 8 spaces instead of tabs)
RWS: fix error when deleting submissions

Commit d89e43b03c468571f08267e26c97e897c3e9ac98 tried to remove a couple
of ugly workarounds but introduced an issue: delete_subchange tried to
access a submission that could already have been deleted. This is now
fixed, but nevertheless the bug showed the shortcomings of the current
data management.
Fix encoding issue in CMS

Pickle was causing issues when a file with non-ASCII characters was
submitted. Since pickle should always output str (both in the ASCII
format and in the binary format), opening the file in binary mode seems
the correct thing to do. And, indeed, it fixes the bug.
Fix bug in CWS

Sometimes downloading a file from CWS (for example a statement)
triggered a 500 HTTP error because a division by zero happened. I don't
know if it's because the timer had a low resolution or because the
transfer did really happen blazingly fast (or because the data was just
written to a buffer), but removing the speed computation surely fixes
the issue.
Use Python's logging system

Abandon our own home-made one in favor of one that uses the `logging`
package of the standard library. It allows more flexibility and removes
some oddities like the double "Logger" definition and some "global"
assignments.
Rewrite ScoringService

ScoringService contained some "oddities". For example: at start it
loaded all submissions from the database, it scored them at a rate of
four each half a second, etc. These were remnants of the ScoreType
Relative, the blocking async loop and the RWS forwarding.

This commit removes all of them, making larger use of the features
provided by gevent. The performance boost is huge.

The RPC API has been left the same, only the internals were changed.

ScoringService has been made contest-agnostic, i.e. it now handles
submissions of all contests simultaneously.
Add support for attachments in YamlLoader