Pre: Get ready to use log in mentat_db.

We don't yet have a logging system for production use, but I'd like to
start experimenting with log, which seems to be (close to) a Rust
standard.  We're already using it in mentat_cli.

db/Cargo.toml

@@ -8,6 +8,7 @@ error-chain = { git = "https://github.com/rnewman/error-chain", branch = "rnewma
 indexmap = "1"
 itertools = "0.7"
 lazy_static = "0.2"
+log = "0.4"
 num = "0.1"
 ordered-float = "0.5"
 time = "0.1"
@@ -31,3 +32,6 @@ path = "../tx-parser"
 # Should be dev-dependencies.
 [dependencies.tabwriter]
 version = "1.0.3"
+
+[dev-dependencies]
+env_logger = "0.5"

tools/cli/Cargo.toml

@@ -13,11 +13,11 @@ test = false
 
 [dependencies]
 combine = "2.2.2"
-env_logger = "0.3"
+env_logger = "0.5"
 getopts = "0.2"
 lazy_static = "0.2"
 linefeed = "0.4"
-log = "0.3"
+log = "0.4"
 tabwriter = "1"
 tempfile = "1.1"
 termion = "1"