Refactoring XPath search request

[traut]

Debugging my code, that heavily uses libtaxii, I discovered that libtaxii throws IndexError when the entity that needs to be inside XML is not there. I refactored most of etree_xml.xpath calls to use get_required, get_optional, or get_optional_text from libtaxii/commons.py. Method get_required will raise ValueError if the required entity is not found

[traut]

I feel I owe some explanation here. Please ignore commit 9b4784a. Commit e6ee837 makes sure that "subscription_id" is treated as optional parameter in PollRequest and refactors how XPath searches are executed

[gtback]

Sorry for not responding earlier, @traut; this is a large change. I hope to have some time to look at it in depth soon.

[gtback]

@MarkDavidson and I took a look at this, and everything looks good. Any pull request which reduces the number of lines of code gets a 👍 from me.