Mappings from R to database types

[richfitz]

Obvious first candidate for support is data.frame, but could use some support for non-data.frame like indexable structures (i.e., lists) and nested lists (access like graphs).

[sckott]

So we have a plain english version of what each DB is doing, in the language of each DB

data.frame's

• couchdb: for each data.frame, create new database, store each row in separate document
• etcd: for each data.frame, create new directory, store each row in separate key
• elasticsearch: for each data.frame, create new index, with type of same name, store each row in separate document

[rfhb]

Covered by documentation in package functions and at https://github.com/ropensci/nodbi#database-connections.