xronos-ch/xronos.R: xronos 0.1.1

This release patches a few bugs in relation to interaction with the current version of the XRONOS API. There are no major changes to functionality.

• Updated documentation to clarify that https://xronos.ch now uses ISO two-character codes for countries.
• chron_data() now uses countrycode to attempt to interpret country arguments that are not country codes.
• Fixed a bug where nested responses from the XRONOS API (e.g. 'typochronological units') could not be collated into a table #5
• Allowed for specifying an alternative API URL endpoint in xronos_request() (primarily for development & testing purposes)