Support multiple rotations within teams

This is a breaking change that changes the interface type for members. This
allows defining multiple rotations within a single team. Useful for teams that
have internally separate sub-groups but want to be accessible externally via
single handles for e.g. the caretakers.

v0.1.1: Fix locale in Docker

The docker image (the recommended method for using take-care) didn't support
UTF8 characters in either input or output otherwise. Non-american could easily
break `list-users`, for example. The locale defaulted to "POSIX", but is now
set explicitly.

v0.1.0 - Initial release

Add project documentation to README