hpx::init should take functions by std::function and should accept all forms of hpx_main

[brycelelbach]

Currently, hpx::init takes only pure function pointers, making it impossible to do things like pass a bound function to hpx::init. Additionally, hpx::init does not accept int(void) and int(int, char**) versions of main().

[hkaiser]

This has been resolved. The main hpx::init overload now takes a int(boost::program_options::variables_map& vm)> and we added an overload taking a function<int(int, char**)>.