Po and pot management utility

Currently the messages template for translations is outdated, specially the references to sources. Also the localizations are outdated. I have implemented a small utility to easily accomplish maintenance task on translations. It encapsulates some `pybabel` functionality, adapted to the Sage Notebook source tree. Also it performs some tasks not directly available from `pybabel`. For example, we can now update translations directly from the source tree without any intermediate template file (`pot`). Only the public API of the `babel` package is used.

The utility has a detailed buitin help. Issue `/path_to_sagenb_source/utility/translations.py -h` on the command line for help. Warnings about the need of manual review on update are issued on demand.

TODO: 
* `sagenb/babel.cfg` is no more neccessary.
* No need to mantain a `sagenb/message.pot`. Updates of existing translations (with manual review, of course) from the source tree can be done, and also new translation initializatiions.
* Update and review of all the translations.
* Changes in the help text.
Trivial typo - this should be `extract` subcommand.