Added Context Manager to Server Class

[RudreshVeerkhare]

This project is really awesome tried this and solved the exact problem I was facing; I'm wondering if you'd be happy with adding context manager support to Server class.
Like currently in documentation, it's given as :

from gcp_storage_emulator.server import create_server

server = create_server("localhost", 9023, in_memory=False)

server.start()
# ........
server.stop()

But after adding context manager support, it'd look like :

from gcp_storage_emulator.server import Server

with Server("localhost", 9023, in_memory=False) as server:
     # .........
     # ........

As this might look like a small change, but this would be really convenient.
Also thanks for such a nice project 😁

[JacobHayes]

Agreed!

[oittaa]

lgtm