Qgrid

- Fixes an issue where the grid widget would flash noticeably when the `df` attribute or any other attributes were changed. This issue has existed since 1.0. (186)
- Make the SlickGrid object available via `$('.q-grid').data('slickgrid')` as suggested here: https://github.com/quantopian/qgrid/issues/178issuecomment-371420892. This allows you to work with a particular SlickGrid instance from the js console.

- Column options can be provided via the `show_grid` method. Options can be provided for all columns via the `column_options` parameter, and for individual columns via the `column_definitions` parameter.
- Added `edit_cell`, `change_selection`, `toggle_editable` methods for updating the state of an existing grid widget without having to call `show_grid`.
- Updated the `add_row` method so that the caller can specify the values for the new row via the `row` parameter. This will allow people to add rows to a qgrid instance even if it's showing a DataFrame that doesn't have an integer index.
- Updated the `remove_row` method so that the indices of the rows to remove can optionally be provided via the `rows` parameter.
- Fixed issue where moving the scroll bar around a bunch of times quickly can cause a series of grid refreshes to occur.
- Thanks to Abigail Hahn ([vardaofthevalier](https://github.com/vardaofthevalier)) for adding many of the features in the following PR: https://github.com/quantopian/qgrid/pull/191

- Fixes issue where object type columns where not being converted to strings before display when they're an index column
- Fixes issue with displaying unicode strings in python 2 (https://github.com/quantopian/qgrid/issues/200)
- Improves support for viewing DataFrames with a MultiIndex.
- Cells are merged vertically (similar to how pandas does it) to make it easier to identify the levels of the index.
- Sorting or grouping any column other than level 0 of the multi-index results in the DataFrame returning to it's normal behavior of never merging cells vertically.
- Column header is hidden for unnamed levels of the index (instead of showing "level_0", "level_1", etc)

- Fixes issue where editing cells in a DataFrame with an unnamed index results in an error pop up (198)

- Make categorical column filtering work with old versions of pandas, like 0.18

- Adds new `on` and `off` methods at both the module-level (for listening to events on all qgrid instances) and at the instance-level (for listening to events on individual qgrid instances)
- The `on` and `off` methods mentioned above should now be used to listen for events instead of listening for the `on_value_change` event of the `_df` attribute. In other words using `qgrid_widget.observe(on_value_change, names=['_df'])` for being notified about changes to the state of the grid (i.e. editing, sorting, filtering) will no longer work.