Unicode-aware handling of non-ASCII object keys in multipart uploads
Add tests of multipart upload with non-ASCII objet key and listing uploads
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).
Use XML export of simple forms of riak_cs_xml:to_xml in multipart uploads
Use XML export of simple forms of riak_cs_xml:to_xml instead of export_xml/1
Fix bug of accept content type
Add test case of listing users which has non-ASCII UTF-8 characters

This is the case of #628.
Fix has already been made by the commit d215335577a3ef02510bc8d115fe187f79eced8c
Add test case of access stats with xml format
Merge pull request #807 from basho/bugfix/non-ascii-in-xml-reorganized

Fix non-ASCII and XML related issues #787 and #628