Add support for postgresql+asyncpg DB scheme

[mhadam]

The supported DBs dict is missing this scheme prefix.

databases/databases/core.py

Line 45 in 6fcb168

"postgresql": "databases.backends.postgres:PostgresBackend",

[aminalaee]

I think the reason behind this is that we have default database drivers:
postgresql --> asyncpg
mysql --> aiomysql
sqlite --> aiosqlite

Alternatives:
postgresql+aiopg
mysql+asyncmy (work in progress)

Is that causing any problems?