support important advice

[awkay]

The madvise system call is actually kind of important in more scenarios that you've covered, and the POSIX values are the same in OSX and linux (I checked .h files in both). The sequential advice, in particular, can have a dramatic effect on the overall VM system (without it you can accidentally kick a lot of stuff out of RAM that is important, unnecessarily).

I added a big docstring to the new method to explain.

The instructions for building didn't work for me...perhaps missing a call to gen the .h file? So, I wasn't able to build it to test, but the change is very small.