0.2.0: Merge pull request #13 from anenriquez/develop

• Fix to_dict and from_dict conversion. Before from_dict was receiving a json_string, now it receives a dictionary and returns an stn object.
• stp: Add method to compute temporal metric.
• dsc: Return the risk level and not the degree of strong controllability.
• Add STNTask class with times relative to a zero timepoint.
• The stn_factory and stp_solver_factory are initialized in config.py and imported in stp.py
• get_subgraph includes a given number of tasks.
• get_task_id based on a given task position and not on a node position.
• update_edge_weight updates the weight if the new weight is less than the previous weight.
• task: Rounds times to two decimals.
• The task_id attached to a node is an UUID object.
• node: Make optional arguments non-optional.
• node: Rename type to node_type.
• MyEncoder: if the object is uuid, return the value of uuid.
• stn: Add method get_idle_time.