Added logging

Added basic logging using Python's logging module.

The logging is configurable via options in the
`config/leonardo.yaml`. There are options for logging to stdout,
files, syslog, and email.

Examples:

* Logging to stdout
```yaml
logging:
  - stdout:
      level: debug
```

* Logging to a file
```yaml
logging:
  - file:
     filename: /path/to/log/file
     level: info
```

* Logging to syslog
```yaml
logging:
  - syslog:
      level: debug
```

* Logging to email
```yaml
logging:
  - email:
     mailhost: example.com
     fromaddr: server-error@example.com
     toaddrs:
       - admin1@example.com
       - admin2@example.com
     subject: example message
     level: error
```