Adding a mozilla adapter. Fix #7 #14 #13

[karlcow]

There is still an issue about the body itself.
Though I'm not sure it is always wise to import all the body as it might not be relevant or useful.
We could do something a bit different for issue #16

[miketaylr]

I'd still like to be able to import a simple JSON file that adheres to the schema. With these changes, it isn't possible:

$ ./import.py issue.json 
Missing --labels or --origin

Is there any reason to not make --origin optional?

[karlcow]

ah! Good point.
So first of all. This pull request was done to finish the thing in a state which was stable and working code. I can modify it again to make possible what you are suggesting. I didn't want to have a half code unfinished, it seemed not right. :p

But my main goal is to ditch the adapters following the discussions we had in #16
which is basically that instead of having adapters, we require a certain format and we import these after checking validity with an additional layer of managing the multiple issues at once. (the responsibility of providing the good format being the people preparing the issues for webcompat).

What do you think?
So what do you want me to do.