Reorganized source files a bit for list and dense and fixed some rake
clobber/rake clean problems. Introduced regression.
Minor bug fixes related to gc registration

 - Added a few missing dtype==RUBYOBJ checks.
 - Fixed a case where a list already freed might be registered.
Fix valgrind invalid reads for list storage

There were two issues:

 - In the set method for list storage, when setting an entry to the default
   value, there was no check if that entry was already the default value before
   trying to delete it.

 - In the nm_list_map_merged_stored function, the storage t would be wrapped in
   a RecurseData object on the stack and then freed later, before the
   RecurseData went out of scope and the destructor was called.  The
   RecurseData tries to unregister t in the destructor (it registers it in the
   constructor) after it has already been freed.  This was changed so that the
   RecurseData is just put on the heap -- kind of hackish.  Better solutions
   welcome.
Merge pull request #9 from cjfuller/list_regression

List regression fixes
Fix additional read after free issues related to registration

Two additional problems cropped up:
 - Some of the list insert/replace methods can free a node while it's still
   registered.  Added a completely_unregister method to get rid of all
   instances of a VALUE in the registration stack.  Use with caution (probably
   only just before something is freed) as this can mess up the registration
   scheme if used improperly.

 - Found an if/else mismatch in list slice_copy where the else was associated
   with the wrong if statement after introducing registration code.  Put the
   else back with the correct if by addition of braces.
Fix registration leak of NMATRIX struct in nm_init_new
Merge pull request #10 from cjfuller/list_regression

Fix additional read after free issues related to registration
Fix possible registration of uninitialized pointer
Merge pull request #11 from cjfuller/list_regression

List regression
Added #potrf, #getrf to NMatrix