Added 'cron' style timed tasks.

Requires the 'croniter' package https://pypi.python.org/pypi/croniter/
Jobs are specified in weather.ini with a '[cron xxx]' section, where xxx
is any name you like. Each has a 'format' entry giving the cron-style
specification for when it should run (in local time). For example:
[cron hourly]
format = 0 * * * *
plot = ['24hrs.png.xml', 'rose_12hrs.png.xml']
text = [('tweet.txt', 'T'), '6hrs.txt']
services = []

[cron daily]
format = 0 9 * * *
plot = ['2014.png.xml', '28days.png.xml']
text = [('forecast.txt', 'T'), '2014.txt']
services = []

The first of these runs every hour, the second at 9am each day. Note
that they may not run at exactly this time. They run after there's new
data from the station. The system will also 'catch up' with a missed
job.