Prepare master for 0.10 breaking changes

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "reqwest"
-version = "0.9.19" # remember to update html_root_url
+version = "0.10.0-alpha.0" # remember to update html_root_url
 description = "higher level HTTP client library"
 keywords = ["http", "request", "client"]
 repository = "https://github.com/seanmonstar/reqwest"
@@ -10,6 +10,8 @@ readme = "README.md"
 license = "MIT/Apache-2.0"
 categories = ["web-programming::http-client"]
 
+publish = false
+
 [package.metadata.docs.rs]
 all-features = true