Add support for tvOS and watchOS

[chrisballinger]

These would first depend on support for these targets in rustup, tracking here: https://github.com/rust-lang-nursery/rustup.rs/issues/1405

Support in cargo-lipo would require changing the "universal" output directory to distinguish between iOS, watchOS, and tvOS libraries.

[seivan]

Depends on this https://github.com/TimNN/cargo-lipo/issues/26