_ennumerate_windows now lists visible only

Added a function flag to only show visible windows.
Also strip null from title strings so now empty titles are an empty
string.
Added wait_for_window() method

Takes the window title as an argument, and loops using the interval
argument until it timeouts, if specified.

If the window exists before timeout, returns True, if it timeouts,
returns False.
_enumerate_windows() no longer prints
Normalize indentation: spaces -> tabs
Added proposed changes.
Default for timeout is 0. Loose title checking.
Merge pull request #8 from gcq/master

Added a wait_for_window() method