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.

src/File.h

@@ -46,7 +46,7 @@ public:
   File& operator= (const File&);
 
   virtual bool create ();
-  virtual bool remove ();
+  virtual bool remove () const;
 
   bool open ();
   bool openAndLock ();
@@ -69,6 +69,8 @@ public:
   virtual mode_t mode ();
   virtual size_t size () const;
   virtual time_t mtime () const;
+  virtual time_t ctime () const;
+  virtual time_t btime () const;
 
   static bool create (const std::string&);
   static std::string read (const std::string&);