implemented working directory hooks!

due to popular demand and general confusion with PilotData et al.:

 PilotCompute.get_local_working_directory()
 # returns a string: /Users/oweidner/agent/bj-e6977047-e66e-11e2-93b6-14109fd519a1

 and

 ComputeUnit.get_local_working_directory()
 # returns a string: /Users/oweidner/agent/bj-554d9178-e66f-11e2-8398-14109fd519a1/sj-55a6dfe3-e66f-11e2-b871-14109fd519a1

These strings can be used to use any out-of-band file transfer mechanisms,
like for example saga-python. For example, you can wait for a CU to
finish, and then do something like:

d = saga.filesystem.Directory("sftp://HOST/%s/" % cu.get_local_working_directory())
d.list()
d.copy('output.txt', 'file://localhost/tmp/')

I will send around a complete example later.