Code Cleanup

- Cmake was not updating HAVE_ST_BIRTHTIME.
- NIBBLER_FEATURE_DATE was not properly applied everywhere.
- FEATURE_COLOR was not properly set.
- Some source files failed to include cmake.h, and therefore were not properly
- Removed inefficient use of std::string::substr for guaranteed single character
  strings.
- Integrated Directory::cd.
- Integrated File::ctime, ::btime.
- Integrated Path::operator+.
- Integrated Nibbler::getDigit{2,4,6}.
- Integrated HighResTimer.
  enabling/disabling code.
- All Path objects now expanded internally to absolute form.
- Modified unit tests to accomodate absolute paths.
- Merged new nibbler.t.cpp tests.
- Made various methods const.
- Includes removed from some files, added to others.

cmake.h.in

@@ -61,6 +61,9 @@
 /* Found timegm */
 #cmakedefine HAVE_TIMEGM
 
+/* Found st.st_birthtime struct member */
+#cmakedefine HAVE_ST_BIRTHTIME
+
 /* Found get_current_dir_name */
 #cmakedefine HAVE_GET_CURRENT_DIR_NAME