Retry XML parsing with huge_tree=True option. Fixes #185.

[stacywsmith]

Upon receiving an etree.XMLSyntaxError exception with a message containing huge text node retry the XML parsing using the huge_tree=True option.

[stacywsmith]

This is one possible way of fixing #185. I only retry the parsing with the huge_tree=True option as needed. However, I'm not sure this is the best option. I'm definitely open to discussion on the correct fix.

[coveralls]

Coverage decreased (-0.5%) to 69.57% when pulling 65c6838 on stacywsmith:issue_185 into 6d781dd on ncclient:master.

[stacywsmith]

@leopoul Before I add unit tests to cover my changes, I'd appreciate your feedback on my approach.

I see three potential ways to address this:

1. Always use the huge_tree=True option.
2. Fall back to the huge_tree=True automatically if parsing fails due to a huge text node.
   (This is the option I implemented in this pull request.)
3. Add a parameter to the manager to allow the user to request huge_tree support.

Thoughts?

[stacywsmith]

I discussed this with a few folks, and I'm now thinking that option 3 in the above comment is a better approach. I'll close this pull request and submit a new one that uses option 3 to fix the issue.