JERSEY-2027: Updating pull request as per the review comments in https://github.com/jersey/jersey/pull/31

- Consolidated jetty.version and jetty.plugin.version properties in POMs.
- Moved all PROXY-* properties to ClientProperties.
- Updated BOM POM to include jetty connector and jetty container modules.
- Jetty Response.Listener.onContent(...) can be invoked multiple times, but Jersey AsyncConnectorCallback.response(...) must only be invoked once. Fixed. Added Async testcase.
- Fixed the wrong property name in localization.properties.
- Removed @author tag from internal class/interface javadoc in ManagedClientTest.