Modify script to do daily summaries of multiple kinds of activities

Since intraday minute by minute info is only available to API partners
I modified things so that this will grab a daily summary of each day
in the given date range.
Proper end date and start date support

If a start date or end date is not specified, we'll use the "memberSince" date
and the last sync date from Fitbit for star and end dates.

Also don't grab data for days after the last sync date - 1 day. This helps
account for days that don't have data or have only partial data. Ex. I sync
at 11am on 2013-08-20 and don't want to record 2013-08-20 until I have the
complete days worth of data and sync on 2013-08-21 or later.
Write user id and account for output file not exsiting yet
Support for fetching user key/secret (use --get_user_keys arg)

Also account for when we're up to date and the start_date is later
than the end_date since we add 1 day to it.
Update README.md