IPMI SEL commands

Implemented commands:

   * `sel info`: `GET :hostname/sel`
   * `sel time set <time>`: `POST :hostname/sel/time {'time': <string>}`
   * `sel time get`: `GET :hostname/sel/time`
   * `sel list`: `GET :hostname/sel/records`
   * `sel elist`: `GET :hostname/sel/records?extended=1`
   * `sel get <record id>`: `GET :hostname/sel/records/:record_id`
   * `sel add <filename>`: `POST :hostname/sel/records {'records': ['%s %s %s %s %s %s %s', ...]}`
   * `sel clear`: `DELETE :hostname/sel/records`

Unimplemented commands due to `pyipmi` deficiencies:

   * `sel delete <record id>`
   * `sel first <count>`
   * `sel last <count>`
   * `sel save <file>`
   * `sel writeraw <file>`
   * `sel readraw <file>`