Header matching option

Hi,

I added this feature to the standalone mode for a project I'm working on and thought it might be useful to contribute back. 

I couldn't find a way to record separate responses for different request headers on the same URL, so I added a command line option to do just that:

    java -jar wiremock.jar --proxy-all http://proxy.com --record-mappings --match-headers Accept,Content-Type

will proxy and record responses, creating separate mappings for requests with different values for the Accept and Content-Type headers.

For example, using the above command, a GET request to http://localhost:8080/some/url with an Accept header of application/json will record a separate mapping than the same request with Accept: text/html.

I also included unit tests for the changes to the StubMappingJsonRecorder class and the CommandLineOptions class, as well as a new acceptance test case for standalone mode.