Modernize top-level makefile.

Implement top level makefile by including the common Makefile.toplevel
and change BUILD_SUBDIRS to all. This avoid errors running make in the
top level when the app subdirectories have been removed (e.g. for released
zip files). Also remove app_example_capsens from TEST_SUBDIRS since it
doesn't have a test target.
Merge pull request #2 from rlsosborne/fix_makefile

Modernize top-level makefile.