Fixed TypeError when running ponysay --onelist.

	Traceback (most recent call last):
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/runpy.py", line 170, in _run_module_as_main
		 "__main__", mod_spec)
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/runpy.py", line 85, in _run_code
		 exec(code, run_globals)
	  File "venv/bin/ponysay/__main__.py", line 154, in <module>
	  File "venv/bin/ponysay/ponysay.py", line 248, in run
	  File "venv/bin/ponysay/ponysay.py", line 765, in onelist
	  File "venv/bin/ponysay/lists.py", line 210, in onelist
	  File "venv/bin/ponysay/lists.py", line 210, in <listcomp>
	  File "venv/bin/ponysay/lists.py", line 104, in _get_file_list
	TypeError: listdir: illegal type for path parameter

This bug was introduced in this commit:

	list.py: Extracted duplicate code for listing files in a directory into a utility function.

I fixed the problem by removing the requirement for lists.onelist() to accept None for directory lists that should not be searched.
Fixed TypeError when running ponysay --altlist.

	Traceback (most recent call last):
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/runpy.py", line 170, in _run_module_as_main
		 "__main__", mod_spec)
	  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/runpy.py", line 85, in _run_code
		 exec(code, run_globals)
	  File "./venv/bin/ponysay/__main__.py", line 154, in <module>
	  File "./venv/bin/ponysay/ponysay.py", line 252, in run
	  File "./venv/bin/ponysay/ponysay.py", line 753, in linklist
	  File "./venv/bin/ponysay/lists.py", line 143, in linklist
	  File "./venv/bin/ponysay/lists.py", line 104, in _get_file_list
	TypeError: listdir: illegal type for path parameter

This bug was introduced in this commit:

	list.py: Extracted duplicate code for listing files in a directory into a utility function.
lists.onelist(): Instead of passing multiple, possibly empty lists, build the list of directories to search before calling the method.