Script to split up large AppEngine backup metadata

This naively, evenly, splits the data files references by a per-kind
.backup_info file (e.g., ProblemLog.backup_info) into two files:
ProblemLog_1of2.backup_info and ProblemLog_2of2.backup_info

It's a good enough solution for ProblemLog, our largest dataset, so it's
good enough for now.

Auditors: jace

Test Plan:
I downloaded the per-kind .backup_info file for ProblemLogs from the
2013-07-17 backup and modified the script to create the first split with
only one data file in it. Then, I uploaded the split file to a test
location in Google Cloud Storage and ran an import into BigQuery using
the AppEngine Backup option. Then I did a "select count(__has_error__)"
for the new table and the number of entries seemed reasonable for one
data file.