Search for project to build if project/workspace are not specified

This is to fix issue [#301](https://github.com/facebook/xctool/issues/301)

If neither -project, -workspace or -find-target are specified, xctool will attempt to find a .xcodeproj to build. An error will display if there are no projects or if more than 1 project is found.

Added some additional test projects and updated unit test to reflect this change.