Silver-nitrate

VCR cassettes recorded with the `vcr_cassette` fixture will now store responses as plain text strings, not binary-encoded data.

Allow periods in the names of VCR cassettes created by `nitrate.cassettes`.

Fix a bug with the JSON encoder where throw a `ValueError: Circular reference detected` error for types which can't be encoded.

Replace the `InMemoryKeyring` class with a new function `use_in_memory_keyring()`.

This avoids keyring trying to use the in-memory keyring when you import `nitrate.passwords`.

Allow passing a custom decoder to `nitrate.types.read_typed_json(…, cls=…)`, similar to the `json.load` method in the standard library.

Initial version.