#111: log physical plan in BSON form

Example output fro `select pop, city as name from zips order by  pop, city`:

```
Mongo
Workflow
|
`- zips
   |
   +- { "$project" : { "pop" : "$pop" , "name" : "$city" , "__sd__0" : "$city" , "order" : { "$literal" : "ASC"}}}
   |
   +- { "$project" : { "__sd_tmp_1" : "$__sd__0" , "name" : "$name" , "pop" : "$pop" , "__sd__0" : "$__sd__0" , "key" : "$pop" , "order" : "$order"}}
   |
   +- { "$project" : { "pop" : "$pop" , "name" : "$name" , "__sd__0" : "$__sd__0" , "0" : { "key" : "$key" , "order" : "$order" , "__sd_tmp_1" : "$__sd_tmp_1"}}}
   |
   +- { "$project" : { "pop" : "$pop" , "name" : "$name" , "__sd__0" : "$__sd__0" , "__sd_tmp_1" : { "0" : { "key" : "$0.key" , "order" : "$0.order" , "__sd_tmp_1" : "$0.__sd_tmp_1"}}}}
   |
   +- { "$sort" : { "__sd_tmp_1" : 1}}
   |
   `- { "$project" : { "pop" : "$pop" , "name" : "$name"}}
```

See #111.