Next potential pull request

[jwrozelle]

@ernestguevarra

Added a few things on an upcoming pull-request based on this branch.

• Added support for downloading multiple versions of ODK briefcase an optional argument in the get_briefcase function to specify which version of ODK briefcase to download and use. There are more elegant ways to do this based on the github api, but for now - I've just added an optional argument to specify the download URL for the desired version. I've thrown in a few stop() and error() messages if the URL looks wrong (like not coming from github, or not ending in .jar). Rationale was that the decryption is kind of finicky depending on source of download and version of ODK. I've unfortunately found I needed to tinker with the version a bit to get it to work properly.
• X11 on linux raised an issue on this separately. Will need to make some changes on the final pull request depending on which decision you make.
• Tests and more tests Up to 67% coverage. Added tests for get_briefcase, pull_remote and export_data. In order to speed up the export_data test, I have added some sample data to inst/testdata, and I added a version of the ODK jar file. You'd know more about whether this would fly with CRAN, I didn't read up on it yet, but, I think we'd need to skip that test on CRAN if we couldn't pre-load the ODK jar because the test for export-data would take too long.