Add an option for generating maps using only tweets within time threshold

[milo-trujillo]

This proposal is related to #29, but does not depend on it: Rather than only collecting tweets within a certain date range (which is sometimes desirable), we can start with a full data sample and limit ourselves to the date range retroactively. This means adding:

• Optional parameters to loadTweetsFromFile with a max age (in epoch time) to load
• Optional parameters to getUserReferences passed through to the loadTweetsFromFile max age
• A new buildAgeMap tool script that builds an RT/Mention network, but takes an argument of the oldest date to build the network from
• Optional arguments for max age to buildRTMap and buildMentionMap so they can also use this functionality

This shouldn't be too complicated an addition, but it's a more sprawling change than #29. No changes to analyze.py or socmap.py