Added a LoadObj function that reads from a std::istream.

 + Added a LoadObj function that reads from a std::istream.  This should allow
   more generic usage and possibly make testing a little easier.
   o This LoadObj accepts a function that returns a
     std::unique_ptr<std::istream> for a material.
 + Modified LoadMtl to read from a std::istream to allow more generic
   usage.
 + Modified test.cc to check that the changes work as expected and nothing was
   broken.

Tests:
 + Compiled test.cc, checked diff of output against pre change output.
   Same output where expected.
Code Modified to remove use of C++11 features.

Tested compilation with VS2012, clang++ 3.4 and g++.
Update README.