Relax type checking of path arguments, part 2.

[tanzer]

Fix more type checks of the form isinstance(cmd, str):

• Replace some of them with checks like::

  not isinstance(cmd, RemotePath)

• Replace others by checking against basestring instead of str.

Unfortunately, back in May, I didn't grep for other occurrences of these
checks. When testing version 1.0 of plumbum, I promptly got::

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "plumbum/local_machine.py", line 531, in __getitem__
    raise TypeError("cmd must be a LocalPath or a string: %r" % (cmd,))
TypeError: cmd must be a LocalPath or a string: u'dir'

Hoping that I fixed everything this time around :-).

Tomer, I'm sorry about reporting this just after 1.0, but could you possibly do a 1.0.1 to fix the remaining type checks?

[tomerfiliba]

thanks @tanzer !