Implement Query in YETI

[MarkDavidson]

Currently, there are not any public examples of doing TAXII Query or asynchronous results. I feel it would be informative to develop these capabilities in YETI (albeit minimally) as a way to demonstrate one possible implementation of Query and asynchronous results. The notional plan for the capability would be to allow queries on IP and File Hash only. One kind of query would always return results synchronously, and the other would always return results asynchronously. Query requests would be fulfilled using the Poll Exchange and Asynchronous Polling, respectively (Note that pushing of results is not in scope). Additionally, add a script to YETI that permits running a query from the command line.

[MarkDavidson]

I updated branch issue_52 with a fix for this. I did the following:

• calls to ContentBlock.content always return a string. This was the previous functionality and (IMO) should be maintained, even if it means that there's an extra serialize every time the function is called.
• the to_etree() function now calls self._content instead of self.content, as self.content now always returns a string. If the underlying content is XML, then it will be stored as an etree which the call to self._content takes advantage of
• If developers want access to the underlying etree, they can use the ContentBlock._content property (which may be an etree or a string, depending on whether the content is XML or not).

[MarkDavidson]

Should have been closed when the dev branch was merged into master.