fix storage mark functions to actually mark ruby objects in object dtype arrays.  Fixes #126.
fixed additional GC marking problems with nmatrix objects of dtype object
change back nm_alloc mark to be NULL
only memset new dense storage for dtype object
remove vestigal recursive gc marking of dense storage src
changed map functions in dense storage to use the rb_gc_(un)register functions to protect storage during initialization instead of the memset to zero hack
revert dense storage creation documentation change
Merge branch 'master' of git://github.com/SciRuby/nmatrix into issue_126

Conflicts:
	ext/nmatrix/nmatrix.cpp
	ext/nmatrix/storage/dense.cpp
	ext/nmatrix/storage/dense.h
	ext/nmatrix/storage/list.cpp
	ext/nmatrix/storage/storage.cpp
	ext/nmatrix/storage/yale/yale.cpp
Merge pull request #128 from cjfuller/issue_126

Fix #126 (segfault in elementwise operations)