Refactor pando-lib to use promises

[corydickson]

We'd like to split these three classes out to separate files if the methods look good and have them bring in the same IPFS/IPLD objects. In the Tree class, the extend() method is omitted because it may not be necessary to have it behave like Commit. There could be something I'm missing tho...

Some improvements:

• [ ] Call extend on each commit of the ordered history (performance issue)
• [x] Use common ipfs && ipld object for making API calls
• [x] Breakout each class into a seperate file

[corydickson]

@osarrouy Thanks for the feedback! Also not sure, I will fix before merge