Fixes for SS when checking for submissions to be scored.

- for new_evaluation, check that the submission is really evaluated;
- use a set for submission_ids_to_* to avoid duplicated;
- do not add to it a submission not evaluated.
Fail gracefully if a wrong contest id is given as command line argument.
Simple ranking facility in AWS.
Referring to the released versions in the README.
Fix bug in UserTest importation
Make AWS accept a 'num' parameter when adding a Testcase
Make YamlReimporter keep user_tests, questions and messages
Restricted localization doc to !cmsranking.
Fix some internationalization issues
Another internationalization fix
Fail gracefully if RS is launched with a non-existent contest id.
More translations.
Suggestions and some bugfixes from pyflakes and pep8.
Mark another string as translatable
Remove wrong TODO (fixes #48).

Sorry, I don't know what I didn't like in this piece of code. Probably
it was just IOI tiredness.
YamlImporter: use n_input for calculating testcases' value.
YamlImporter: fall back to Sum ScoreType if there is no gen/GEN (fixes #51).
Missing piece to a previous commit.
YamlImporter: when using Sum, make the total task value configurable.
Avoid getting score_type once per submission in AWS/user.html.