Handle well-indented XML including comments

The root cause for #782 was misuse of xmerl_scan:string/2,
which doesn't emit spaces, comments between actual xml elements.
It leaves them as #xmlText{} and #xmlComment{} - This commit
changes to ignore them at walking around parsed dom tree. This
diffset also includes a small refactoring that wraps all
xmerl_scan:string/2 to one riak_cs_xml funcsion to control its
usage.

This also adds regression test as simple eunit test case.

Conflicts:
	src/riak_cs_acl_utils.erl
	src/riak_cs_s3_response.erl
Merge pull request #795 from basho/bugfix/gh782-acl-xml-format-1.4

Handle well-indented XML including comments