Buildslave fires SIGTERM before SIGKILL

Before forcefully killing the process, we fire a SIGTERM in case
the developer wants to capture some debugging information.
Fixes trac ticket #751.
Updates test_simple to check if process was killed by SIGTERM
Adds test_kill to buildslave tests

Adds a unit test which ensures the process will receive SIGKILL if
SIGTERM fails to end the process.
Updates relnotes to reflect new behavior

States that processes will receive a SIGTERM before
a SIGKILL
Merge branch '751-fire-sigterm-before-sigkill' of git://github.com/markberger/buildbot