Allow specifying windowsSdkDir on VisualCpp

Added windowsSdkDir property to be configured for the VisualCpp
toolchain and extended DefaultVisualStudioLocator to support Visual
Studio 2012.
- See http://forums.gradle.org/gradle/topics/how_can_i_configure_the_default_search_path_for_the_visualcpp_tool_chain for more details.