Define in more exact terms what interface store.open() should return

[criemen]

At the moment, Backends return different interfaces for open(), for which the API docs just say that it "[r]eturns a read-only file-like object for reading a key."
Boto for example does not provide a seek method, depending on the backend wrong seeks are not always throwing an exception, etc.
An example test run can be seen here, which included the azure tell-and-seek-test for all backends:
https://travis-ci.org/Corni/simplekv/jobs/301494259
It would be nice to have this as a minimal file-like interface which is supported by all backends.