fix the `was compiled for iOS Simulator, but linking for MacOSX` warning

We're seeing ...

```
ld: warning: ObjC object file
(/Users/fpotter/Library/Developer/Xcode/DerivedData/xctool-coaqfpnavanndzgumiqgxmfwpkjd/Build/Intermediates/otest-query.build/Debug/otest-query-ios.build/Objects-normal/i386/main.o)
was compiled for iOS Simulator, but linking for MacOSX
```

Where we used to pass `-miphoneos-version-min=7.0` in Xcode 4, we now
need to pass `-mios-simulator-version-min=7.0`.
adding a `-showTasks` option which logs info on all spawned tasks.

When `-showTasks` is passed, xctool will log a message for every NSTask
that is spawned.  Included in each message is short description of the
task's purpose and a command-line expression that can be copied/pasted
into bash to replicate that command.

The output looks like --
https://gist.github.com/fpotter/6981876

I've wanted this for a long time - it's going to be super helpful for
debugging problems.
Expected assertion failures are now silent.