Fix whitespace
Fix comments
Configure webhook URL instead of token

The Slack webhook URL has changed. It doesn't contain the company name
anymore. The full URL is used because it can be copied easily from the
Slack webhook configuration.
Update README.md
Merge pull request #12 from uschek/change-webhook-url

Use webhook URL instead of company name and token
Add some sed magic to escape double quotes and backslashes properly for the JSON submission to slack
Fix a couple typos in the post-receive-hook.

I should have tested more BEFORE I made the pull request.  But now I have tested it and fixed these bugs.