use .jsreport extension for export file

[bjrmatos]

we should use ".jsreport" extension for the file produced after export, this will make it more clear that the format of the file is designed specifically for the import/export flow, and it is not mean to be consumed or extracted as a normal zip file.

this will require other changes in the messages, logs, cli args, modify the file extensions for drag and drop

[bjrmatos]

we have decided that the file extensions we are going to use will contain the ".jsr" prefix, so the extension for this case should be now ".jsrexport"

[pofider]

Implemented in jsreport 3 beta