Doc new behavior of simctl_reset
Adds spec Resource methods for CoreSimulator

Need a way to inspect the CoreSimulator directory
New specs for simctl_reset
Add sim_udid argmument to simctl_reset

Can now erase individual simulators.
Docs new behavior of reset_sim_content_and_settings
New spec to test reseting individual simulator

Xcode 6 only test.
Reset sim can now target individual sim
Fix spec grouping in core spec
Add specs for Core.default_simulator
Adds Core.default_simulator (see details)

Calabash iOS needs to know what the default run-loop simulator
is so calls to reset-app-sandbox can target an individual
simulator vs. reseting the sandbox of every device.

ATM, the Calabash::Cucumber::Launcher #reset_app_sandbox cannot
reset an application's sanbox; it can only erase the targeted
simulator's content and settings.
Use default sim in Core.udid_and_bundle_for_launcher
Merge pull request #39 from calabash/feature/reset-individual-simulator

SimControl can erase individual Xcode 6 simulators