Fix issue where qgrid would prevent all other jupyterlab widgets from working

[TimShawver]

• Fixes issue where qgrid prevents all other widgets from working when used on Jupyterlab (Qgrid install breaks bqplot in jupyterlab #154, ipywidgets stop working after enabling qgrid #165)
• Adds new sortable and filterable grid options which allows users to disable sorting and/or filtering for the entire grid (disable sorting / filtering #164)
• Adds new highlightSelectedCell and highlightSelectedRow which allows users to change the cell/row highlighting behavior. Setting highlightSelectedCell to True and highlightSelectedRow to False will result in spreadsheet-style cell highlighting (in response to Twitter feedback https://twitter.com/Tim_Shawver/status/957850647172395008)
• Convert the values in categorical type columns to strings before serializing to json
• Use a constant for PAGE_SIZE because I didn't mean for it to be configurable, and I'm not sure qgrid's scrolling will work well if people change this
• Add processPhosphorMessage to handle resize events in Jupyterlab and when qgrid is embedded in other widgets. (qgrid is not correctly resizing when it is unhidden inside of an accordion widget #169, JupyterLab Widget doesn't resize #167)
• Trigger _df_changed event for filtering changes and edits. (Observing "_df" does not trigger when a cell is changed #170)
• Fixes a minor issue where the search text wouldn't be cleared in the text filter when the reset button is clicked
• Bumps the version to 1.0.1