Workaround to allow dynamic building on MSVC

#6

I have no clue why it works like this, but this change allows it to build. Maybe you can figure it out better than me from the following links.

http://stackoverflow.com/questions/767579/exporting-classes-containing-std-objects-vector-map-etc-from-a-dll
stackoverflow.com/questions/4145605/stdvector-needs-to-have-dll-interface-to-be-used-by-clients-of-class-xt-war
http://stackoverflow.com/questions/11189662/warning-c4251-needs-to-have-dll-interface-to-be-used-by-clients-of-class