Specs for issue #152 and issue #153, fix for issue #153

The list storage slice_set method never incremented v_offset, so when setting a
(non-single) slice, every value would be set to the first value in the rhs
array.

The specs are now segfaulting on list stype, which I suspect is related to
Merge branch 'issue_152' into gc_management

Conflicts:
	ext/nmatrix/storage/list.cpp

Some lines had changes both related to issue 153 and using NM_ALLOC.  These now
incorporate both changes.
list matrix gc and bug fixes

- Implemented gc registration for list matrices.  Still needs more extensive
  testing on more complicated code.

- Fixed a few bugs related to iterating on slices by reference for list
  matrices.  Within the list code, some locations do recursion starting with 0
  and counting up; others do it starting with dim-1 and counting down.  Offsets
  needed to be adjusted accordingly; previously some locations were using slice
  offsets for dimensions ordered in reverse.

- Fixed #152 by matching sype during iteration.

Some stat-function-related list and yale bugs remain, but these can be dealt
with separately.

(Sorry for the multi-bug commit, but I needed to fix all of these at the same
time for all the specs not to fail or segfault, and I needed the specs to make
sure I hadn't introduced regressions.)