Add std feature to Cargo manifest

This enables use of this crate in environments where std is not
available.

Author: richard-w

Cargo.toml

@@ -12,17 +12,25 @@ version = "0.2.3"
 readme = "README.md"
 build = "build.rs"
 
+[package.metadata.docs.rs]
+features = ["std"]
+
 [dependencies]
-num-traits = "0.2"
 proc-macro2 = "0.4.2"
 quote = "0.6"
 syn = "0.15"
 
+[dependencies.num-traits]
+version = "0.2"
+default-features = false
+
 [dev-dependencies]
 num = "0.2"
 
 [features]
+default = ["std"]
 full-syntax = ["syn/full"]
+std = ["num-traits/std"]
 
 [lib]
 name = "num_derive"