Simplify ScopeGuard usage and allow arbitrary return types

Since `Callable` is required to take no arguments and ignores the return type it can simply take `std::function<void()>` as it's argument and wrap other function types using `std::bind` (allows for move semantics for lambda captures in C++11).

Because the constructor is now a template instead of the class you can eliminated the need for `auto` and `make_scope_guard` and simply use the default constructor (less copying this way and looks cleaner in my opinion).

All previous use of `make_scope_guard` will still work.

New Syntax:
```
scope_guard guard([&] { value = true;});
// also supports
scope_guad guard([&] { value = true; return 1; /* any return will work */; });
```


Tests are passing on OSX (XCode, Clang)