Changing dependency structure of packages to make more intersting

I also added a test-only dependency and test-only library to demonstrate.

This is now better example project.
Add TESTONLYLIBS and IMPORTEDLIBS to TRIBITS_ADD_EXECUTBLE() (TriBITS #24)

With this commit, now TRIBITS_ADD_EXECUTBLE() takes new arguments TESTONLYLIBS
and IMPORTEDLIBS and deprecates DEPLIBS.

In addition, this now provides strong checks that TESTONLYLIBS and
IMPORTEDLIBS don't contain the wrong entires.  I need to add some automated
testing for this but I don't have the test setup to do this yet.
Fixed up to use TESTONLYLIBS