Use editable_row_callback to determine if row should be editable

[TimShawver]

This is a branch off of the following PR: #191

My reasoning for using a callback function is it's more flexible/powerful and fewer lines of code for us to maintain.

Also I'm thinking ahead to other similar features which we'll want, like have the ability to highlight rows/cells with a background color based on the contents of the rows. For those cases we'll definitely want the ability to do stuff like highlight all rows where an integer is less than zero. With a callback function we can handle that type of comparison, because we'll have the full power of python available to us in the callback, rather than being limited to a subset that we've added support for in the row_edit_conditions grammar.

There's one other small tweak here which is to make it so that columns default to being editable if the editable attribute is not specified when constructing the QgridWidget. This is only an issue when constructing QgridWidget directly so you wouldn't have noticed it if you've just been testing with show_grid.

[TimShawver]

@vardaofthevalier sorry about the delay in merging this, I was busy with making qgrid available on Quantopian (see https://www.quantopian.com/posts/qgrid-now-available-in-research-an-interactive-grid-for-sorting-and-filtering-dataframes) and also wanted to some more time to think about how the "setting cell value" piece should work. I'm wondering if we should re-use the editing code path that already exists for editing a cell by double clicking. I'll update this PR with my thoughts once I have some more time to think about it.