Create ~/.gitshots if it doesn't exist. Dump data to json. Optionally don't post anywhere. README improvements.

A few miscellaneous improvements.

1. Create the `~/.gitshots` directory if it doesn't exist. This isn't the most robust way to test this, but it will work for the common case.
2. Dump the data to `~/.gitshots/timestamp.json` for later use.
3. If the environment `GITSHOTS_SERVER_URL` is set to `False` don't post the image at all, just save it locally.
4. Added more details to the README for client side setup, particularly that `requests` and `imagesnap` are required.