Add useful __repr__() for DB and PrefixedDB

Fixes issue #16. Example output:

>>> db
<plyvel.DB with name '/tmp/testdb' at 0x0x2243b78>
>>> db.prefixed_db(b'foo')
<plyvel.PrefixedDB with prefix 'foo' at 0x0x234ad10>
>>> db.close()
>>> db
<plyvel.DB with name '/tmp/testdb' (closed) at 0x0x2243b78>