calabash should enable accessibility on all 'possible' simulators

## motivation

Previously, calabash could enable accessibility on any simulator _for which a target had been built for_.

Put another way, if the

```
 ~/Library/Application Support/iPhone Simulator/Library/7.0.3-64/Library/Preferences/com.apple.Accessibility.plist` 
```

existed, accessibility could be enabled _before_ launch.

This PR tries to  deduce the the list of all 'possible' simulators based on the installed SDKs and the Xcode version and to pre-emptively write a `com.apple.Accessibility.plist` with accessibility enabled.

## tests

```
$ be rake spec

```

Also tested with Briar, but the test is not automated (or published) yet.