Removed need for xml-rs and log in projects using yaserde

[oscartbeaumont]

This is achieved by exporting xml-rs and log from the yaserde crate which is already included in projects using yaserde.

Then xml-rs can be accessed by ::yaserde::xml instead of ::xml and log can be accessed by ::yaserde::log instead of ::log.

This fixes issue #22 and is an improvement to the developer experience of using this crate.

[azzamsa]

+1, I always need to do additional step such adding xml_rs and log just to use yaserde

[MarcAntoine-Arnaud]

Very cool feature ! thank you @oscartbeaumont