Fixes a JSONKeystore bug.

This commit fixes a bug whereby an instance of JSONKeystore created with
the null constructor would not update its in-memory store after a
keystore file was attached (e.g. when using Spring beans). An extra
bit of debug logging was also added.
Removes the unnecessary `encryptionKey` property.

The `encryptionKey` on TokenAuthenticationHandler is no longer
necessary. The key is automatically looked up via the token_service
parameter.
Cleans up documentation and fixes a bug.

This commit simplifies the example in the documenation. It also fixes
a bug that would prevent a valid authentication from succeeding due to
a verification against an invalid "embedded key". I'm not entirely happy
with the final check for a valid token (by simply checking the username)
but it will do.
Adds a CLI tool to create an encrypted token.

This commit adds a simple tool that can be used to craft an encrypted
token for test purposes. It also includes a class that can be used
independent of the CLI tool for encrypting and decrypting strings (could
be useful in a REST interface).