Adds filtering to servlet request messages

[jscottnz]

This PR adds data filtering ( inspired by #31 )

While the ability to filter messages has been available for sometime with a roll your own implementation of RaygunOnBeforeSend, this PR adds some out of the box implementations:

• RaygunRequestQueryStringFilter to filter out query string values
• RaygunRequestHeaderFilter to filter out header values
• RaygunRequestFormFilter to filter out form values

ie.

RaygunClient.SetOnBeforeSend(new RaygunRequestFormFilter("form1", "form2"));

And the bonus: RaygunOnBeforeSendChain which allows specifying a set of RaygunOnBeforeSend handlers:

RaygunClient.SetOnBeforeSend(new RaygunOnBeforeSendChain()
     .filterWith(new RaygunRequestQueryStringFilter("queryParam1", "queryParam2").replaceWith("*REDACTED*"))
     .filterWith(new RaygunRequestHeaderFilter("header1", "header2"))
     .filterWith(new RaygunRequestFormFilter("form1", "form2"))

[mduncan26]

LGTM