Allow using plyvel.DB as a context manager

[Necklaces]

Allows for using with on plyvel.DB, like this:

with plyvel.DB(...) as db:
    ...

I'm not too familiar with the codebase, but I added a test and it seems to work.

[wbolster]

thanks, lgtm mostly. left a few comments.

note that you can already do this today using contextlib.closing() from the python stdlib

[Necklaces]

Neat, I might've been a little too fast so those were great suggestions.

I amended the commit with those changes to keep history clean

[wbolster]

thanks, lgtm. merged