Add -SPT_setUp and -SPT_tearDown to run global cleanup/teardown

These methods are similar to OCUnit's `-setUp` and `-tearDown`, which are run between every spec, except that the `SPT` versions are run between every example (e.g., each `it` or `itShouldBehaveLike` block).

This is useful for global set up/teardown work that needs to happen for all tests. For example, in GitHub for Mac, we use this opportunity to put clean repository fixtures into place for each test.

@joshaber