Implemented enhanced plotting options for histogram

*log y
*limit by cuts)
Make histograms more "histogram-y", with square steps
Merge branch 'log-hist' of https://github.com/eteq/ginga into eteq-log-hist
Updates to eteq's Histogram changes

- made plugin set the GUI checkboxes for new feature's state over
   repeating opening and closing of the plugin
- made the plot update when the user cuts the levels while the plugin is
    open
- made "Plot by cuts" the default setting