Rename background decorator to async.
Hide Future class constructor and make callable and its arguments to Future attributes.
Make Pool instance valiables private.
__init__ method should not return anything but None.
Pool.kill method should not do anything when called more than twice.
Change local variable name of Pool.__run.

Name `future' instead of `task'.
Make documents detail.
Fix bug of sample code.
Modify comment for pep8.
Restrict error at exit in particular condition.
Add how to install from pip to README.rst
Merge branch 'devel'