Automatically compute download counts in CircleCI nightly runs

[cboettig]

Tasks for the parse_s3_logs.R script:

• Download log files from AWS (needs Fix automatic downloads of log files from AWS  #2 fixed)
• Parse raw log files (and append to previously parsed records in log.csv file)
• Tidy the resulting data.frame into the format used by RStudio (anonymized IP, etc)
• Write out the tidy, anonymized data.frame to downloads.csv
• Push downloads.csv to GitHub or public S3 subdirectory for sharing / other applications
• Push the parsed log.csv back to S3
• Delete the already parsed & captured logs from S3

Then add a call to source("parse_s3_logs.R") to build.R.

[cboettig]

okay, all the necessary methods are now implemented and tested in aws.s3, and tested locally. Testing on circle builds now.