Ignore Scheme in database URI

As per the discussion in the mailing list, the ecto:// scheme in the database URL makes it a bit tricky for people using Heroku (for instance), where their configuration only provides them with a URL starting with mysql:// or postgre://. However, as ecto doesn't do anything with the scheme actually, there is no impact in ignoring the scheme altogether.

The PR here ensures a scheme is present (using roughly the same regex as per the URI RFC document) but the parsing doesn't fail anymore if the URL doesn't start with ecto://.

I left a TODO for someday, do something with the scheme (such as ensuring it's part of a list schemes ecto can actually understand), but I assume this isn't really the priority.