Modified interface of gwt20 to make it easier to test

Currently, testing Atmosphere GWT is very difficult because everything is a concrete implementation with no interface provided.

This commit fixes this problem by providing various new interfaces for Atmosphere, AtmosphereRequest, AtmosphereResponse and AtmosphereRequestConfig.

This avoids a lot JSNI as well.
Rework the contribution to prevent incompatible API change. Proper changes for RequestConfig will happens once 2.1 is released
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