Vendor dependencies

Let's see how I like this workflow.

vendor/cassowary/Cargo.toml

@@ -0,0 +1,25 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g. crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+name = "cassowary"
+version = "0.3.0"
+authors = ["Dylan Ede <dylanede@googlemail.com>"]
+description = "A Rust implementation of the Cassowary linear constraint solving algorithm.\n\nThe Cassowary algorithm is designed for naturally laying out user interfaces using linear constraints,\nlike 'this button must line up with this text box'.\n"
+homepage = "https://github.com/dylanede/cassowary-rs"
+documentation = "https://dylanede.github.io/cassowary-rs"
+readme = "README.md"
+keywords = ["constraint", "simplex", "user", "interface", "layout"]
+license = "MIT / Apache-2.0"
+repository = "https://github.com/dylanede/cassowary-rs"
+
+[dependencies]