Add Unicode handling of lists in the same manner as binaries

The change of riak_cs_xml:format_value/1 affects only the calls of
riak_cs_xml:to_xml/1. All lists, binaries and integers of XML elements
are created in the module riak_cs_xml privately.

On the other hand, this change does not affect calls of
riak_cs_xml:export_xml/1.
Generarize riak_cs_xml:to_xml/1 to convert xmerl's simple form

Add function clause to treat xmerl's simple forms.
http://www.erlang.org/doc/man/xmerl.html#export_simple-2

This function internally uses riak_cs_xml:format_value/1 so
atoms, intergers, lists and binaries are automatically converted
to lists (of Unicode codepoits, which xmerl accepts).