obj: Fix tabs causing vertices to be skipped

If an obj vertex definition 'v' is followed by a tab instead of a space,
the vertex definition is skipped.
obj: Add support for relative vertex indexing

obj files allow faces to specify indices relatively by using negative
integers. See vertex refering in http://www.martinreddy.net/gfx/3d/OBJ.spec
Merge pull request #138 from calvindhsu/master

OBJ: Add relative indexing support