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/shadow.t

@@ -43,16 +43,16 @@ if (open my $fh, '>', 'shadow.rc')
 }
 
 my $output = qx{../src/task rc:shadow.rc add one 2>&1 >/dev/null};
-like ($output, qr/\[Shadow file '\.\/shadow\.txt' updated\.\]/, 'shadow file updated on add');
+like ($output, qr/\[Shadow file '.+\/shadow\.txt' updated\.\]/, 'shadow file updated on add');
 
 $output = qx{../src/task rc:shadow.rc list 2>&1 >/dev/null};
-unlike ($output, qr/\[Shadow file '\.\/shadow\.txt' updated\.\]/, 'shadow file not updated on list');
+unlike ($output, qr/\[Shadow file '.+\/shadow\.txt' updated\.\]/, 'shadow file not updated on list');
 
 $output = qx{../src/task rc:shadow.rc 1 delete 2>&1 >/dev/null};
-like ($output, qr/\[Shadow file '\.\/shadow\.txt' updated\.\]/, 'shadow file updated on delete');
+like ($output, qr/\[Shadow file '.+\/shadow\.txt' updated\.\]/, 'shadow file updated on delete');
 
 $output = qx{../src/task rc:shadow.rc list 2>&1 >/dev/null};
-unlike ($output, qr/\[Shadow file '\.\/shadow\.txt' updated\.\]/, 'shadow file not updated on list');
+unlike ($output, qr/\[Shadow file '.+\/shadow\.txt' updated\.\]/, 'shadow file not updated on list');
 
 # Inspect the shadow file.
 my $file = slurp ('./shadow.txt');