Make clippy an optional dependency

ethcore/Cargo.toml

@@ -18,7 +18,7 @@ ethcore-util = { path = "../util" }
 evmjit = { path = "../evmjit", optional = true }
 ethash = { path = "../ethash" }
 num_cpus = "0.2"
-clippy = "0.0.41"
+clippy = { version = "0.0.41", optional = true }
 crossbeam = "0.1.5"
 lazy_static = "0.1"
 
@@ -27,3 +27,4 @@ jit = ["evmjit"]
 evm-debug = []
 json-tests = []
 test-heavy = []
+dev = ["clippy"]

ethcore/src/lib.rs

@@ -17,9 +17,8 @@
 #![warn(missing_docs)]
 #![feature(cell_extras)]
 #![feature(augmented_assignments)]
-#![feature(plugin)]
-// Clippy
-#![plugin(clippy)]
+#![cfg_attr(feature="dev", feature(plugin))]
+#![cfg_attr(feature="dev", plugin(clippy))]
 // TODO [todr] not really sure
 #![allow(needless_range_loop)]
 // Shorter than if-else