Almost ready for a 2.0 release

Cargo.toml

 [package]
 name = "rustyline"
-version = "2.0.0-alpha"
+version = "2.0.0"
 authors = ["Katsu Kawakami <kkawa1570@gmail.com>"]
 description = "Rustyline, a readline implementation based on Antirez's Linenoise"
 documentation = "http://docs.rs/rustyline"