Allow scheduling boost::function

[wch]

This makes it simpler to schedule functions like the ones mentioned here:
rstudio/httpuv#106 (review)

@jcheng5, before merging, we should discuss the object lifetime issues that came up in an earlier conversation. I don't think that they will be a problem here.

I think we avoided adding this in the past because of concerns that it would introduce a boost dependency, but I don't think that this PR is actually adding such a dependency.

[wch]

In order for code from another package to schedule a boost::function, the later.h file will need a later() implementation that can take a boost::function. This also means that we'll need to add #include <boost/function.hpp>, and that will require packages that use later to have the BH package installed at build time. This is why we didn't implement this earlier.

I think it should be possible to do this by providing something like an abstract class or interface that doesn't require boost, but I don't know for sure.

[jcheng5]

Implemented using a subclass instead