Save the user maps from layer0 to disk

[milo-trujillo]

Similar to #3. Once we have dictionaries of which seed users are mentioning whom, and which seed users are retweeting whom, we need to save the results as a GML graph to disk. Specifically we want a directed graph of seed user retweeted/mentioned layer 1 user saved as layer1.gml

This should be saved in the mapdir ("./map" by default). Each node representing a user should contain the following information:

• username
• screen name
• twitter ID
• layer
• retweeted (true if the user is included because they were retweeted, false otherwise)
• mentioned (true if the user is included because they were mentioned, false otherwise)

[milo-trujillo]

Implemented and pushed. So far I only have a subset of the above information:

• username
• layer
• retweeted
• mentioned

If we decide we really need the rest we can add it later and make another issue.