Merge pull request #28 from raedwulf/master

Fix for user stencil bug on MacOS X's OpenCL
fixes #15
Allow users to set program headers and build options globally

vex::set_program_options(const cl::Device&, const std::string&);
vex::set_program_header(const cl::Device&, const std::string&);
Stencil operators should work on all platforms

refs #15
Allow to set same header/build options for all devices in queue list
Test user-define program header