Allow dots in property names.

This is for vCard compatibility. From RFC 6350:

    The group construct is used to group related properties together.
    The group name is a syntactic convention used to indicate that all
    property names prefaced with the same group name SHOULD be grouped
    together when displayed by an application.  It has no other
    significance.  Implementations that do not understand or support
    grouping MAY simply strip off any text before a "." to the left of
    the type name and present the types and values as normal.

We could of course consider stripping off the group prefixes, but then
that data would be lost. I recommend we leave this to the user or
Component subclasses to do something meaningful with groups.
Merge remote-tracking branch 'untitaker/vcard_groups'
allow dots in property names