Operators, algorithms, VCDB example, ASN Timeline example

* Adds the `operators` module, which includes
    * `difference()`
    * `intersection()`
    * `union()`
* Adds the `algorithms` module, which includes:
    * `diff()`
* Added several methods to the core `Graph` class, including:
    * `copy()`
    * `remove_nodes()`
    * `get_node_ids()`
    * `remove_edges()`
    * `get_edge_ids()`
    * `add_nodes()`
    * `add_edges()`
    
    as well as some other changes:
    * Allow JSON objects in the constructor
* Added examples: VCDB, ASN Timeline. Changed mergers to use logarithmic scaling