Extend support to Visual Studio >= 2010

There were only a few minor hiccups preventing the current project to be compatible with all MSVC versions since 2010 (before 2010 there was no stdint.h). This patch:

 - Compiles code as C++ (this is only needed to get a true bool type. If replaced by int, this is unnecessary). Wish I knew how to make this conditional on MSVC version; in 2013, this switch (set in sodium.props) can be removed without any ill effect.
 - Changed VS project type to 2010. Later VS versions will simply upgrade the project (and toolset) to their respective versions automatically.
 - Removed some unnecessary inttypes.h inclusions, and put stdbool.h inclusions inside #ifdef guards.