Add feature: exporter.  Provides csv and pdf exporting.  Draft only

This code is almost entirely standalone - it all resides in the exporter feature.  There are two exceptions/potential impacts:

1. A minor extension to the selection feature to expose the "getSelectedRows()" method on the publicApi, allowing export of selected rows.
2. The inclusion of pdfMake into the dependency list if people want to export pdfs.  This might have implications for the ui-grid.info build, and implications for bower packaging.  My thought for bower packaging is to not declare this as a formal dependency, and require people to manually install if they want to use this feature - otherwise everyone will end up installing pdfMake even if they don't use it.

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Commit message:

Add exporter as a feature, with associated directives.

Provides for CSV export and PDF creation using the pdfMake library.
Includes tutorials and ngdocs.

No UI is currently provided, the tutorial provides an example of
building your own UI.  Ideally this would be added to a grid menu
once that function is available.