Cipher text (and plain text in the /encrypt endpoint) can now
carry a prefix of {name:value} pairs, all of which are passed
into the TextEncryptorLocator (instead of just the app name
and profiles).
The inputs are prepared symmetrically by the EncryptionController
(if used) and the EnvironmentEncryptor (used in the EnvironmentController).
Tidy up docs and add notes on keys.