Refer forked xmerl to avoid many undef exceptions thrown

Original issue: #669 

Add forked xmerl to deps and refer https://github.com/shino/xmerl/pull/1 .
Xmerl source is based on Kelly's fork and updated to R16B02's.

- Pull request of xmerl change on erlang/otp: https://github.com/erlang/otp/pull/78
- Similar change is also made by Richard Carlsson: https://github.com/erlang/otp/pull/87

Once either of these PRs is merged and Erlang/OTP is released,
we don't need forked one any more.