Fix bugs with color lists and place-image-align.

Color lists now work correctly, and exhibit the same behaviour that they
do in Racket. Part of this includes changing the definition of a natural
number from n >= 1 to n >= 0, to match Racket's definition.

place-image-align had a minor bug in the JavaScript code that caused it
to error out. It has been fixed and now works correctly.
unwrap exception to avoid over-wrapped final exns
do this in the right place so resuming contexts can pass an exn with existing stack info
pyret built
Give triangle drawing functions same behaviour as HtDP.

Functions for drawing triangles now have the same behavior as those in
the HtDP teachpack. Redundant lookups in the color database have also
been removed, since those are handled by checkColor.
More work on list examples
Hmm, converter turns lam to fun and _ to $underscore
Make list examples actual tests, resugare lists in tests
Merge branch 'new-world' of github.com:brownplt/pyret-lang into new-world
now that the concrete syntax of s-lams has changed, its tosource ought to change to match...

(cherry picked from commit e9e12c716d128a9479f92ad72d0d26dcd2a0a574)
fixing the edge case of reconstructing parse trees where a rule has a
right-nullable suffix consisting of multiple nonterminals and all of
them were empty, for constructUniqueParse and constructNextParse.
ConstructAllParses already handled this case...

(cherry picked from commit 02bedf8eb388d2570f54f5262fd31b01137deb52)
Merge branch 'new-world' into spinnaker

Conflicts:
	build/phase0/pyret.js
	src/js/base/runtime-anf.js