Adds removeFeedWindow(), adds "-" button

removeFeedWindow() function creates new window, populates it with items found in urls. removeFeedWindow() is called when "-" button is pressed.
Improves remove button, adds removeFeed function

Fixes and improves the way the remove button and the way the refresh button works. Adds the removeFeed function, creates a new window and populates it with all the feeds found on file. Clicking a button removes the feed from the urls list and skips it when writing to file. This removes the entry in sites.txt.
Fix globals in refreshRSS(), Change file to f

Globals were in the wrong order, this caused errors. Fixed. Changed file to f due to builtin Python name.
Merge pull request #20 from powderblock/remove-feeds

Add ability to remove feeds.