extracting acceptConnection so that it can break.

removing 'forever' and flatten a function.
using a variable for readability.
moving comments and mask_.
refactoring.
breaking according to the result of accept().
refactoring.
refactoring according to #282.
Adding connection counter.
adding a new API: setInstallShutdownHandler.
comments.
adding a missing module.
more doc.
Merge branch 'graceful-shutdown'