Refactored the exporters into plugins; Currently only the CSV plugin exists, and is selected by default.
Basic version of a XLS exporter using POI
Added some basic styling to the excel exporter
Merge remote-tracking branch 'upstream/master' into exporter_plugins

Conflicts:
	wicket-pivot/src/main/java/ro/fortsoft/wicket/pivot/PivotTableRenderModel.java
	wicket-pivot/src/main/java/ro/fortsoft/wicket/pivot/exporter/PivotCsvExporter.java
	wicket-pivot/src/main/java/ro/fortsoft/wicket/pivot/web/PivotPanel.java
Rename IPivotExporter -> PivotExporter, and use plural for the PivotExporters
Also perform the renames in the exporter package.
Merge pull request #6 from rototor/exporter_plugins

Exporter plugins