Get extra connection args from query string in db_url
Looks good, just one thing to change and I'll be happy to merge.
 Actually that dictionary comprehension breaks in Python 2.6 (forgot about that). I didn't notice it before because travis didn't pick up the PR initially.
 A few fixes were required for Python 2.6.

### Changes
- As discussed in #858, this adds support for supplying extra connection args in the query string when using playhouse.db_url
