Support for HTTP DELETE requests with a body

The `HttpDelete` class used by BrowserMob Proxy does not support HTTP DELETE requests with a body. This pull request adds support for them.

There appears to be [some disagreement](http://stackoverflow.com/a/299696) whether this type of request is valid according to the [W3C specification](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html). That said, applications in the wild do it, browsers support it (or at least don't prohibit it), so it probably makes sense that BrowserMob Proxy supports it, too.

The new `HttpDeleteWithBody` class is based on [`HttpPost`](http://www.docjar.com/html/api/org/apache/http/client/methods/HttpPost.java.html). The source of this idea is this [Stack Overflow question](http://stackoverflow.com/a/3820549/581722).