added a "make shell" target to spin up ipython with a db and a session already configured to talk to the test database. Handy for poking around.

added a chunksize configuration option for S3 backups so that sqlalchemy doesn't fetch all rows into memory when journalling a backup file

first crack at reworking smaller tasks for S3+SQL backups

reworked the s3+sql backup tests so that zip files are logged through
heka

refactored the s3+sql backups again so that smaller subtasks are
scheduled when the write_measure_s3 task is invoked.
dropped start/end id for blocks as we already pass the block_id
dropped chunk_size configuration option from s3 backup
write out column names immediately after the csv backup file is opened
added a filter so that deletion of CellMeasure and WifiMeasure records
will not occur if any measure record associated with a block is too
recent.
moved test for measurements which are recent enough that we shouldn't bother deleting them yet
cleaned up a lot of argument passing and pushed some configuration into ichnaea.models and loading other bits directly from the block
broke out delete measure records into a celery subtask
Merge pull request #223 from crankycoder/features/212-smaller-s3backup-tasks

Features/212 smaller s3backup tasks