Simultaneous futures compat (#172)

This patch adds opt-in support for futures 0.2.

Cargo.toml

@@ -27,6 +27,7 @@ members = [
   "tokio-io",
   "tokio-reactor",
   "tokio-threadpool",
+  "futures2",
 ]
 
 [badges]
@@ -44,6 +45,7 @@ mio = "0.6.14"
 slab = "0.4"
 iovec = "0.1"
 futures = "0.1.18"
+futures2 = { version = "0.1", path = "futures2", optional = true }
 
 [dev-dependencies]
 env_logger = { version = "0.4", default-features = false }
@@ -60,3 +62,12 @@ time = "0.1"
 
 [patch.crates-io]
 tokio-io = { path = "tokio-io" }
+
+[features]
+unstable-futures = [
+    "futures2",
+    "tokio-reactor/unstable-futures",
+    "tokio-threadpool/unstable-futures",
+    "tokio-executor/unstable-futures"
+]
+default = []