Added unit tests for #73 columns

@manns In response to your comment on #73, here are some unit tests for the columns.  Wanted to check in with you and make sure I was in the right direction before doing the same for rows.  A couple of concerns I have:
- Is this test located in the right place?  I was thinking maybe it should be in test_main_window_actions as there is where it gets bound, but it seemed to make more logical sense to place it with your existing resize testing logic.
- Is there a more desirable way to create a fake event than to use an event test harness class like I have?
- The second to last test fails.  Apparently you can't have decimal values of width (width is 15 instead of 15.15)?  I see you are doing so in `test_set_col_width()` but I notice that test is not touching any of the wx Widget methods, which may prohibit setting the width to decimal in the actual interface?
- The last test fails.  Apparently you can't set a column to 0 width?  Again, I see you did this in `test_set_col_width` which passes...