New Racket version 5.3.1.12.
Disambiguate sentence on custodian shutdown.

See:
http://lists.racket-lang.org/dev/archive/2012-December/011373.html

Motivated by Sgeo's question in:
http://racket-lang.org/irc-logs/20121229.txt at 22:10.
fix handin server error on bad password
adjust drracket window size and position preferences to use a different key

This will make switching back and forth between 5.1.3 and the latest
not lose the frame size and position preferences (since the format
of the data is different now that the current release remembers
the position on a per-monitor-configuration basis)
avoid compiler warning
Added back partial pivoting to Gaussian elimination
whoops, fix a bug introduced in 0f9674e1ddd598a97bec61a9e759102182a26789
fix `expt' on small negative number and large positive odd

The pow() function apparently gets it wrong on some platforms.

Closes PR 13391
check the result of find-ls in the one place it wasn't being checked

probably closes 13404
racket/gui gtk: make `on-demand' for menus work better

There appears to be no way to make `on-demand' work right for the
Unity window manager's global menu bar, since there's no notificiation
when the menu bar is clicked. We approximate the correct behavior
by calling `on-demand' when a containing frame loses the keyboard
focus, which might be because the menu bar was clicked; that may be
too late (because the menu has already been shown), but it should
work most of the time.

Closes PR 13347.

Relevant to PR 13395, but DrRacket will have to change to work
around the remaining limitations of `on-demand'.
fix doc typo

Closes PR 13353
revise README and "Getting Started" info

Addresses PR 13356
Make let: annotations optional.
Fix tests.
`expt' repair, again

Previous repair wasn't general enough; negative powers can also
go wrong.
Made Gaussian elimination faster (half the scaled additions) and more
accurate (always produces zeros in the lower half)
Initial `math/matrix' documentation, and some changes

* At least stubbed out all (or almost all) `math/matrix' exports; many
  have complete documentation (e.g. types, predicates, accessors,
  constructors, for loops, conversion, much of "Comparing Matrices")

* Moved `matrix-zero?' and fixed to use (matrix-error-norm) as its norm

* Added `matrix-basis-cos-angle' (currently a stub; should return smallest
  singular value of a certain matrix multiplication)
Adjust DrRacket to not use on-demand to populate the
teachpack menu.

Instead DrRacket explicitly changes the menu items when
the language changes or when a teachpack is added/removed

Also, Rackety.

Closes PR 13395
adjust sync pattern to avoid cross-thread set!
added section on alpha blending to 2htdp/image guide