Adds --log=LEVEL to allow specifying verbosity.

[dlzr]

Hey folks,

I need to run adb-sync from a cronjob, and any output from the command will generate an email. I need it to only print output in case something went wrong, to minimize the number of spurious emails.

adb itself writes its normal chatter to stdout, which I redirect to /dev/null, and only error messages end up in stderr, as expected. adb-sync uses python's logging library, which seems to write everything to stderr, regardless of severity, which makes it hard to only silence the benign verbosity.

So, this PR adds a --log flag, to make it possible to silence the "everything is fine, just doing my job" messages, while leaving the serious stuff on. I've set it up to default to INFO, which is what the current code is doing.

I've already signed the CLA.