Adds new feature to allow user to toggle filters on and off

This pull request contains changes to implement this feature: https://github.com/x-team/wp-stream/issues/329

- Adds a row of checkboxes which when checked, will hide the corresponding filter controls above the table.

- For now, the admin page is refreshed every time a setting is updated. It might be preferable to reload the filter controls via an Ajax call if possible. ( maybe call `filters_form()` again ). 
Another option instead of updating the page on each checkbox check, we could add a button "Toggle Filters" which would update all at once and refresh the page.

- Adds a new user meta entry to store the user preferences.

- Moves the 'Live Update' settings into the same function as these controls which filters the `screen_options`.

![screenshot 2014-03-20 11 32 50](https://f.cloud.github.com/assets/6961890/2472976/97b46dc4-b03c-11e3-8196-310f898b07a4.png)

- [ ] Could add a message if no filters are selected: "Add filter contriols via the screen options tab above"