Device class can provide instruments ready simulator names

**RunLoop::Device needs a method to return 'instruments ready' names for simulators** #51

```
moody@stern:~/git/run-loop
|2.1.3| (feature/device-class-can-provide-instruments-ready-simulator-names-issue-51)
$ irb
Don't touch that button!
irb(main):001:0> sim_control = RunLoop::SimControl.new
#<RunLoop::SimControl:0x007fb85a386390>
> sim_control.simulators.map { |device| device.instruments_identifier }
[
    [ 0] "iPhone 4s (7.0 Simulator)",
    [ 1] "iPhone 5 (7.0 Simulator)",
    [ 2] "iPhone 5s (7.0 Simulator)",
    < snip >
    [19] "iPad Air (8.1 Simulator)",
    [20] "Resizable iPhone (8.1 Simulator)",
    [21] "Resizable iPad (8.1 Simulator)"
]
> xcode_tools = RunLoop::XCTools.new
#<RunLoop::XCTools:0x007fb85a41cf98>
> xcode_tools.instruments(:devices).map { |device| device.instruments_identifier }
[
    [0] "e60ef9ae876 < snip > b79e5606d",
    [1] "43be3f89d95 < snip > a1bd91124",
    [2] "44f1fdea2446< snip > 20633b0db"
]
```