Require user to supply absolute path to input file

[Peter-Metz]

Previously, the input file for the Batch class was specified with a path relative to taxcrunch/multi_cruncher.py. For simplicity and consistency, the input data will now be specified with an absolute path to the file. For example:

CURRENT_PATH = os.path.abspath(os.path.dirname(__file__))
input_path = os.path.join(CURRENT_PATH, "example_input.csv")
b = Batch(input_path)

Documentation/tests are updated to reflect this change. Does not affect web app.