Always print include dirs and lib dirs path, update include dirs msg (TriBITS #41)

Now ${TPL_NAME}_INCLUDE_DIRS and ${TPL_NAME}_LIBRARY_DIRS are printed even if
they are empty.

Also, I make it explicit that headers are being searched for in
${TPL_NAME}_INCLUDE_DIRS.
Setting defaults for TRIBITS_DIR and HeadersOnlyTpl_INCLUDE_DIRS

Now, if you configure this project right out of the soruce tree, it will set
${PROJECT_NAME}_TRIBITS_DIR and HeaderOnlyTpl_INCLUDE_DIRS by default.  This
makes it easier for people to try configuring TriBITSExampleProject with a
simple:

  cmake $TRIBITS_DIR/doc/examples/TribitsExammpleProject