Unfiltered snapshots feature - #139

This pull request removes the wasFiltered flag (the source of a number of bugs), and replaces it with a _latestUnfiltered state object. The purpose of this object is to track the most recent unfiltered state, in order save the correct state object to the past at the correct time (not too early).

The basic premise:
-When updating new (unfiltered) present state, push _latestUnfiltered to the past stack (if it exists), and update both the present and _latestUnfiltered state objects with the latest state
-When inserting filtered present state, merely update the present state
-When undoing, push the _latestUnfiltered state to the future stack, and update both the present and _latestUnfiltered state objects to the most recent past
-When redoing, push the _latestUnfiltered state to the past stack, and update both the present and _latestUnfiltered state objects to the nearest future

I've also updated the unit tests to account for this; let me know if you have any questions/concerns

Example functionality changes

Actions:
Action A = unfiltered action
Action F = filtered action (typically fired by a thunk for redux form's internal state)

Before this pull request:
1: Action A happens
2: Action F happens <--- line 1's state is added to past (Undo will have to be clicked 1 too many times!)
3: Action F happens
4: Action A happens
5: Action F happens <--- line 4's state is added to past
6. Action A happens

After this pull request:
1: Action A happens <--- snapshot of "most recent unfiltered state" is taken
2: Action F happens
3: Action F happens
4: Action A happens <--- line 1's state is added to past; snapshot of "most recent unfiltered state" is taken
5: Action F happens
6. Action A happens <--- line 4's state is added to past; snapshot of "most recent unfiltered state" is taken