Fill in simulation methods

[gjtrowbridge]

This is effectively a rebase of my original pull request, and adds implementations for the basic setter/getter methods.

In addition, this diff also moves the async-specific logic away from the Simulation struct and onto the Handle struct. I believe the Simulation struct should not be tightly coupled to the web server, or the async logic, etc. so it can remain as flexible as possible, so it can be more easily unit-tested, and so it's interface is not influenced by our current choice of async library, etc.

[SamWilsn]

bors try

Please try and run cargo fmt before putting up the pull request. It'll keep a consistent style, and make subsequent diffs less noisy.

[bors[bot]]

try

Build failed

• continuous-integration/travis-ci/push