Added msgpack_vc2008.sln, msgpack_vc2008.vcproj from msgpack-0.5.8.tar.gz.

And removed them from .gitignore.

I tried to upgrade msgpack_vc8.* files to Visual Studio 2012. This failed miserably. Then I downloaded msgpack-0.5.8.tar.gz and discovered that an upgrade of msgpack_vc2008 seems to work fine. Some sleuthing revealed that msgpack_vc2008.* were part of .gitignore brought  by the commit e511c32a3630 (Imported .gitignore from the original MessagePack git repository).