Import useful classes in mycroft package

[MatthewScholefield]

This is open for discussion, but I think it is worth importing a few classes/functions in mycroft/__init__.py so that skills can do things like:

from mycroft import MycroftSkill  # vs. from mycroft.skills.core import MycroftSkil
class MySkill(MycroftSkill):
# ...

Or:

from mycroft import FallbackSkill, play_wav
class MediaFallback(FallbackSkill):
    def handle_fallback(self, message):
        # ...
        play_wav('my_song.wav')
        # ...

[forslund]

I think this is an excellent idea!

Message is another class that's dreadfully long to include if one wants to use:

currently

from mycroft.messagebus.message import Message