Compatibility with Microsoft compilers

This pull request adds support for Microsoft Visual Studio 2010 and 2012. 

Changes:
 - Added a VS project on dist-build/msvc
 - Some minor changes to enable MSVC compilation (where possible these are guarded by #ifdef _MSC_VER)
 - Added the SODIUM_STATIC macro. If this macro is defined, then SODIUM_EXPORT is empty and static linking is assumed. Otherwise, SODIUM_EXPORT is __declspec(dllimport) and dynamic linking is assumed. This only matters when using MSVC.

To maintain compatibility with the above changes, the regular autotools build system is only required to define the empty macro SODIUM_EXPORT.