Add an optional keepRaw property

which stores the raw parsed result on the reader instance for use by the application. A lot of times an application needs more than what can be expressed in the records of a store. This property allows an application to access the raw parser result from the OpenLayers format.

See also https://github.com/boundlessgeo/gxp/blob/master/src/script/plugins/WMSSource.js#L24:L55 for a bit of background.

TIA for any review. Tests are included.