Support Servlet API 3.0

When deploying cxf 3.2 on tomcat 7, it breaks because of a hard dependency on Servlet 3.1 classes in Servlet3ContinuationProvider (javax.servlet.WriteListener), which results in ClassNotFound at runtime. At some point JAX-RS 2.1 API dropped NIO support and the dependecy on Servlet 3.1 API as well, right before  releasing the spec (commit reference https://github.com/jax-rs/api/commit/a4faa965bbe4d058caa41f56f712fa52fc9600fe#diff-bb05b5ab9309760c3271445278116a2f).

The Servlet 3.0 should be the base line.