add nested object support

[juliangruber]

This adds nested object support, as in https://github.com/dominictarr/level-manifest/issues/1

For a db with:

typeof db.nested == 'object'
typeof db.nested.get == 'function'
db.methods == {
  nested: {type: 'object', methods: {get: 'async'}}
}

the manifest is:

manifest(db, true) == {
  nested: {type: 'object', methods: {get: 'async'}}
}

I'll be working on a patch for multilevel to support this too.

[dominictarr]

cool, merged into 1.2.0

you don't need to put the tidying up things into separate commits. if it's straightforward housekeeping you can put those all into one and there is less things to click on.

[juliangruber]

ok will do :)