Julia v0.7 support

This release adds v0.7 compatibility (v0.6 no longer supported) and introduces the following major breaking change:

JULIA_CPP_MODULE_BEGIN and JULIA_CPP_MODULE_END no longer exists, define a function with return type JLCXX_MODULE in the global namespace instead. By default, the Julia side expects this function to be named define_julia_module, but another name can be chosen and passed as a second argument to @wrapmodule.