OEmbed: use micawber providers in their latest version

As micawber has been released with a new version which contains all
providers in our local list, it's pointless to maintain a list
ourselves. Instead, use the micawber basic list by default.

The local list of FLUENT_OEMBED_PROVIDER_LIST is now empty (BIC only if
you defined FLUENT_OEMBED_SOURCE = "list" explicitly in your settings).

There is a FLUENT_OEMBED_EXTRA_PROVIDERS setting instead which extends
the available providers, regardless of the source.
Make sure the OEmbed type can never be used to control filenames.

Minor risk, as it's still a template path, but better be safe then sorry.
Fix updating OEmbed code after changing maxwidth/maxheight parameters.
Support running 'setup.py test' too.
Added tox.ini