Project updates for header file references

So you don't have to read through a million lines of Xcode project files, I'll give the overview of what I've done. :)

- At the core of this review, I've changed the way that projects reference the headers of their dependencies.  It used to be that we would set header search paths right into the source path of a dependent project.  This approach, while functional, didn't give us much chance to dogfood the way dependent projects would work in the wild.  So now, projects reference their header dependencies by the output path of the public headers that gets created when the dependent project is built.

- I also took the opportunity to change `#import` references to dependent projects, to be of the form `#import <SalesforceSDKCore/SFAuthenticationManager.h>`, to better call out that the import is being leveraged against a different project.  That's essentially all of the changes to the .h and .m files that you'll see here.

- I did a whole bunch of miscellaneous clean-up of project configurations as part of this too—better to get all of that cruft out of the way at once.  This was mostly things like standardizing where library references are defined in test projects, where header and library paths are set, etc.  It was fairly haphazard from project to project.

- The xcscheme file changes are where I added explicit project dependencies to the Xcode configuration, so that it would know to e.g. build SalesforceOAuth before trying to build SalesforceSDKCore, and so on.

- Finally, the build had to be retooled, because, while Xcode can understand the project dependencies mentioned above, apparently xcodebuild cannot.  So the dependency handling now happens in the ant build script instead.

All in all, not much of interest to see here.  A little reconfiguration and a whole lot of housecleaning.