cleanup: comments and unused code

Mostly comment cleanup, plus deleted one unecessary line.

	modified:   src/filebuffer.rb
	modified:   bufferhistory.rb
cleanup: created FileAccessor class for interacting with files.

This class handles read, write, etc for the FileBuffer class, and helps
lighten its load.

	modified:   filebuffer.rb
feature: better cursor position after undo/redo

Cursor moves before changes now.  Makes it easier to see the changes
occuring, since the cursor doesn't leave the scene.

	modified:   bufferhistory.rb
	modified:   filebuffer.rb
bugfix: call better cursor position whether text changes or not.

	modified:   filebuffer.rb
Feature: indentation string swapping using object ids

Now we swap indentation strings in all of history, so that undo/redo
works with indentation facade.

	modified:   bufferhistory.rb
	modified:   filebuffer.rb
Speedup: shorten saved history length for large files

Set a history length (number of snapshots) limit *and* a number of
characters limit.  This way large files will have fewer snapshots
saved, and won't take as long to process a save command.

One thing is left untested: will this cause problems for really large
files, where the file size is larger than the size limit?  Should
probably generate a very large file and see what happens.

	modified:   src/bufferhistory.rb