Merge branch 'release-0.4.9'
Added a function collapseCell so extenders can access at that level.
Added getValue(), getRows(), getCount() to ExportData.
Added Exporter::cssSafe().
HTML table cells now have class colgroup-COLUMN_NAME
Removed extra line break in MarkdownTableExporter.
Added BootstrapHTMLExporter
Version bumped from 0.4.9 to 0.4.10
Merge branch 'release-0.4.10'