Add function for redcap_download_report()

[wibeasley]

From API documentation

Method Name: Export Reports

Description: This method allows you to export the data set of a report created on a project's 'Data Exports, Reports, and Stats' page.

Note about export rights: Please be aware that Data Export user rights will be applied to this API request. For example, if you have 'No Access' data export rights in the project, then the API report export will fail and return an error. And if you have 'De-Identified' or 'Remove all tagged Identifier fields' data export rights, then some data fields might be removed and filtered out of the data set returned from the API. To make sure that no data is unnecessarily filtered out of your API request, you should have 'Full Data Set' export rights in the project.

Also, please note the the 'Export Reports' method does not make use of the 'type' (flat/eav) parameter, which can be used in the 'Export Records' method. All data for the 'Export Reports' method is thus exported in flat format. If the 'type' parameter is supplied in the API request, it will be ignored.