letsencrypt renamed "certbot"

[Leo Famulari]

The Let's Encrypt client (letsencrypt [0]) was recently renamed to
certbot and upgraded to 0.6.0. [1]

I renamed the letsencrypt package to certbot and applied the update in
9495cf9a8, keeping a letsencrypt variable that we can phase out later
on.

Please note that the executable's name has changed to 'certbot'.

[0]
https://github.com/letsencrypt/letsencrypt

[1]
https://pypi.python.org/pypi/certbot