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.

test/color.err.t

@@ -46,8 +46,8 @@ if (open my $fh, '>', 'color.rc')
 # Test the errors colors
 my $output = qx{../src/task rc:color.rc rc.debug:on add due:__ 2>&1 >/dev/null};
 like ($output, qr/^\033\[33mThe\ duration\ '__'\ was\ not\ recognized\ as\ valid,\ with\ correct\ units\ like\ '3days'\.\033\[0m$/xms, 'color.error');
-like ($output, qr/^\033\[32mTimer\ Config::load\ \(color.rc\) .* \033\[0m$/xms, 'color.debug');
-like ($output, qr/^\033\[34mUsing\ alternate\ .taskrc\ file\ color.rc\033\[0m$/xms, 'color.header');
+like ($output, qr/^\033\[32mTimer\ Config::load\ \(.+color.rc\) .* \033\[0m$/xms, 'color.debug');
+like ($output, qr/^\033\[34mUsing\ alternate\ .taskrc\ file\ /xms, 'color.header');
 like ($output, qr/^\033\[31mConfiguration\ override\ rc.debug:on\033\[0m$/xms, 'color.footnote');
 
 # Cleanup.