Extend HttpLogClient fucntionality. 

Add get STH and get root certificates methods in HTTPLogClient.
Also, one of the reasons the HttpPostInvoker exists is that it can be mocked in the tests, enabling you to test the class only through its public API without making any actual network requests. Have a look at:
https://github.com/google/certificate-transparency/blob/master/java/test/org/certificatetransparency/ctlog/comm/HttpLogClientTest.java#L78