Added a 'glob' command to the slave.

This slave command can be used to retrieve a list of files from the slave that match a given pattern. (This is somewhat more portable than using shell expansion.)

Custom steps could use this command to add filenames to a property of the build where they may be later picked up by an uploading step.

For example:

```python
cmd = buildstep.RemoteCommand('glob', dict(path='[Bb]uild[Bb]ot-*', workdir='wkdir'))

def setProperty(_):
    g = cmd.updates["glob"][-1]
    self.setProperty('artifacts', files, self.__class__.__name__)

d = self.runCommand(cmd)
d.addCallback(setProperty)
```

This way it can be neatly combined with #969:
```python
transfer.MultipleFileUpload(slavesrcs=Property('artifacts'), masterdest='somewhere')
```