Feat/load from zip

[ComicallyNormal]

Hi. I am working on a project downstream of this sdk, that would need to utilize loading from an xmind file and interpreting data from it. Specifically, we will be using trees that will be compared against a valid schema, using custom markers to validate that nodes are at the level in the tree that they are supposed to be at.

I have updated the feature branch with code from master, and implemented basic loading functionality for this project. I would like some feedback, especially about any problems that you guys have run into in the past in attempting to implement this functionality.

Please do not merge this immediately, there are some architectural problems ( the biggest of which is that my loading functions are in the Topic class) that I would like to sort out before considering this ready to merge.

Thank you for your time, and apologies if I am approaching this process wrong, this is my first stab really at open source.