Support lowercase 'ns' and 'ni' obj material tokens

Previously, the obj material parser would interpret all key tokens
starting with 'n' as 'newmtl' tokens. However, there are other tokens
that can start with 'n', namely 'ns' and 'ni' for material properties.
Make the parser robust to such cases, despite their canonical forms
being uppercase.

Resolves #410.
Merge pull request #413 from jdduke/fix_ns_obj_material

Support lowercase 'ns' and 'ni' obj material tokens