Twitter field extractors, custom output template module, and a unit testing framework for publisher modules

### Twitter extractors
All key-path accessible payload keys from Twitter Activity Streams have been defined in a new module `twitter_acs_fields.py`. Each value is a accessible via an object that defines the path through the JSON payload to the appropriate value, which is stored in an attribute of the object. The `twitter_acs_fields.py` module handles the checks for existence of keys and stores a default value if the destination value isn't reached. This means the construction of the output for e.g. `gnacs.py -z twitter` is much cleaner.

### Custom output (Twitter only)
The `acscsv/` directory now includes a `custom_output.py` module that can be used for one-off projects with custom delimited output. To keep the master branch clean, the desired workflow is to pull all upstream changes to master, create a new branch, and `mv` or `cp` the `custom_output.py` module into one that is named appropriately for the task. The `CustomCSV` class inherits from the core `AcsCSV` class, so all of the helper methods and machinery are available to the custom class. For customized output fields and order, the developer only has to fill out the `get_output_list()` method defined in the new copy of the `custom_output.py` module. In this way, the developer can maintain a branch that includes any of their one-off projects, delete it at any point, or simply cherrypick from master when there are substantial changes there.  

### Unit testing
New modules have been added to the `acscsv/` directory to implement Python's unit testing framework on each of the publisher-specific modules. For now, `unittest` boilerplate has been added to all modules, except for native-format Stocktwits, which includes ~10 test methods. The remaining modules should be filled out soon by other contributors.