Create object library class

[spencerahill]

Copying from https://github.com/spencerahill/aospy/issues/156#issue-215307459, since it's not the main topic of that issue:

One idea: create an ObjectLibrary class, something like:

class ObjectLibrary(object):
    def __init__(self):
        self.projects = {}
        self.variables = {} 

I definitely want to implement this (although not necessarily with the exact API above). Among other things, the examples section of the docs, which gets compiled from aospy/docs, is unable to load the example_obj_lib module located in aospy/aospy/examples, without a hack-ish step of cd to the examples directory.

More generally, it just makes sense to have an actual class to represent this core data structure, rather than just having it be a module or package upon which we just require certain attributes.