Fixes for VS2012 build
- Correct version of .sln format so it opens in VS2012 instead of 2010 when double-clicked.
- Ignore .user files created by VS.
Merge pull request #28 from Reedbeta/vs2012-build-fixes

Fixes for VS2012 build
Merge branch 'master' of https://github.com/mmp/pbrt-v2