Use the right URL for the project

`lets-encrypt.org` doesn't work but `letsencrypt.org` does so use that.
Merge pull request #113 from fmarier/url-typo

Use the right URL for the project
Random small changes
Merge remote-tracking branch 'github/letsencrypt/master' into pep8

Conflicts:
	letsencrypt/client/client.py