Allow locales/ from framework to be used on framework clients

[alvarogzp]

To do so:

• the locales/ dir must be marked to be packaged on setup.py
• the .mo files must be generated either on setup.py before packaging, or on clients (at install? or while generating theirs?)
• the internationalization.py file must support it

Maybe not needed if we remove extras package, as suggested in #107, and delegate all i18n translations to clients. We can provide a basic telegram-bot.po file with translations to be copied to clients.

I think the latter is preferable over the former, because that way clients can modify freely translations, and are not tied to a framework-provided ones, that might not adjust to their needs, or might not be translated to all languages they need.

If we go that way, the things to do are:

• inform in README that users should copy locales/ dir and generate them
• do not package locales/ dir (currently we are not doing it)
• once Split working bot to a separate repository #107 is fixed, remove all non-applying locale info (if any translated code is moved to the new repo)
• update_po.sh?