Rename settings.json to settings.json.example, ignore settings.json

[rgoliveira]

Since said file (settings.json) could be adjusted by someone running the bot, it doesn't really make sense to have it on version control. We should, however, keep an example of it (always updated, if possible), aka settings.json.example.

Similar thing happened to tokens.json, although this one was created to move tokens out of settings.json. When we deal with this, maybe we could just move the tokens back into the main settings?

[rgoliveira]

One problem to consider: current tests (read Travis CI) uses settings.json in its instance