MSVC 11 expects __declspec(dllexport) before return type.

This fixes compilation on MSVC 11 (2012) Express. Apparently the compiler expects __declspec(dllexport) first, before the return type. For some reason this was only generating errors on lines 45 and 96 (the const char** return types), but I changed it everywhere in these two files (context.cpp and opentrack.h) for consistency.  

Please test against earlier/later versions of MSVC, since 11 is all I have installed. I may add MSVC 2013 soon, will test against that then.