Encrypt content sent to the server

This implements client-side encryption, so that users' task information
is not availble to the server (or to anyone who does not have the
`encryption_secret`).

taskchampion/Cargo.toml

@@ -20,6 +20,7 @@ kv = {version = "^0.10.0", features = ["msgpack-value"]}
 lmdb-rkv = {version = "^0.12.3"}
 ureq = "^1.5.2"
 log = "^0.4.11"
+tindercrypt = { version = "^0.2.2", default-features = false }
 
 [dev-dependencies]
 proptest = "^0.9.4"