Allow default runner to be configured via command line/env vars

[modocache]

QuickCheck/Fox isn't performant--that's not what it was designed for, after all. So when running Fox locally, I probably only want to test each property a few hundred times, in order to keep my test runs short and my feedback cycle tight. But when running on CI, or for nightly builds, time isn't as much of an issue anymore, and I may want to test each property thousands of times.

FOXRunner should be configurable via the command line, so that my CI servers can run more exhaustive tests, whereas I only run the default amount.

[jeffh]

This should be available, but xcodebuild test does not pass environment variables through to the test bundle for iOS. (See global configuration section of http://fox-testing.readthedocs.org/en/latest/runner.html). I think a new strategy needs to be devised.