Allow sequential runs of killable targets

Configuring a simple shell target like:

``` js
shell:{ echo:{ command: 'echo beep'}},
...
grunt.registerTask('repeat', ['shell:echo', 'shell:echo']);
```

then running it with

    > grunt repeat

Dies with

    Fatal error: Process :echo already started.

Even though the first echo finished before we tried to start the second.  

Tested and fixed in this PR.  Tests depend on #10 for tests.