[leo_gateway]Replace xmerl with something not generating atom

[mocchira]

Since xmerl produces new atoms for every element name/name spaces, We need to look for another alternative for safe.

Candidates

• exml
• erlsom

[yosukehara]

I've modified the milestone:

• Write its documentation with v1.3.5
• Fix it with v1.4.0

[mocchira]

Another choice is to

• Keep using xmerl and specify the appropriate XML Schema through validation and schemaLocation described at http://erlang.org/doc/man/xmerl_scan.html#type-option_list