Adds sigtermTime to RemoteShellCommand

Adds the parameter sigtermTime to RemoteShellCommand. If sigtermTime is
not None, the buildslave will fire SIGTERM and wait for sigtermTime
seconds before firing SIGKILL. This fixes trac ticket #751.
Adds test_sigterm and updates tests to reflect new parameter
Documentation updates for sigtermTime
Adds sigtermTime default to fake runprocess
Waiting for sigterm is now nonblocking
Check slave version before supplying sigtermTime param
Updates relnotes
Fixes version checking
Increment version and update relnotes
Merge branch '751-fire-sigterm-before-sigkill' of git://github.com/markberger/buildbot