Fix for dplyr 0.7.0 updates and others

• Fix attempt to set an attribute on NULL error for anti-sample
• Fixed anti_sample="auto" math to actually limit the number of sessions for the auto-batches
• Add warning if using default project, and if API quota is tripped (#79)
• refactor defensive checks from testthat to assertthat
• Fixed error when no data present in requested date_range for a view, and max = -1
• Fix logic of anti_sample_batches to allow accurate day batches. (#74)
• Fix parsing of ga_account_summary broken by upgrade from dplyr 0.5.0 > dplyr 0.7.0