BUG: conversion of datetime instance to format required in GET request fails
if usgs.eddn.get_data encounters a 'Max Data Limit Reached' message,
it will now make multiple requests until all available data is retrieved.
tweak docstring for get_data
BUG: mocking with multiple urls/files requires that we set
match_querystring=True when calling register_url
Merge pull request #72 from twdb/feature/eddn-long-time-periods

usgs.eddn.get_data will now make multiple requests if it encounters a 'Maximum data limit reached' message.