Clarify test/example cmake settings.

This commit attempts to fix issue #92 via the following changes:

* Rename BUILD_TESTING_{STATIC,SHARED} to
  BUILD_TESTS_AND_EXAMPLES_{STATIC,SHARED}, everywhere.
* In the comments for BUILD_TESTING and BUILD_EXAMPLES, mention the
  importance of BUILD_TESTS_AND_EXAMPLES_{STATIC,SHARED}.
* Edit comment for BUILD_TESTS_AND_EXAMPLES{STATIC,SHARED}.
* Add CMake errors if the user wants tests or examples but sets both
  BUILD_TESTS_AND_EXAMPLES_{STATIC,SHARED} off.