Switch to wasm-timer

[tomaka]

Close #900

The wasm-timer crate abstracts over tokio_timer and std::time::Instant and make them work in the browser.

We replace all usages of tokio_timer and Instant with this crate.

This might be controversial, as this change is quite invasive.
However, once #1029, #1070, and this PR are all merged, it should be possible to run a WASM example that uses the master branch.

[twittner]

Could the wasm-timer crate make use of WASI and we generally support the wasm32-unknown-wasi target? Maybe a little early and dependent on the timeframe for WASM support you have in mind.

[tomaka]

While the idea of WASI seems cool, it doesn't really look ready to me yet (example: https://github.com/rust-lang/rust/blob/bfb443eb1de484fde141fa9090a9f4291cbe60a5/src/libstd/sys/wasi/net.rs#L14-L106).
We additionally would need some support from browsers to actually implement WASI, and that doesn't seem to be in the process of happening.