local.cwd doesn't work if cwd changes by non-plumbum means

[henryiii]

This patch makes the local.cwd the current working directory, not just the current working directory when Plumbum loaded, so code like this now works:

assert local.cwd == os.getcwd()
os.chdir('../plumbum')
assert local.cwd == os.getcwd()
os.chdir('../tests')
assert local.cwd == os.getcwd()

The cwd is now a descriptor in the local machine (and works from class or object access). Added a test similar to the lines above to verify that it works.

(Example of use case: This is a common problem when working with Plumbum in IPython and trying to use local.cwd to find files after changing the directory with cd.)

[tomerfiliba]

Looks good. I'd just call _FunctionDescriptor a StaticProperty.

[henryiii]

Done. Moved it to lib.py, too.