Implement Mango selectors for change feeds

API is modeled after _doc_ids filter for change feeds.

User POSTs to {db}/_changes with `filter=_selector`.
Document body should have a "selector" field, with
a Mango selector object as value.

For example:
```
http http://.../d1/_changes?filter=_selector
{
  "selector": {"z" : {"$gte" : 1} }
}
```

Jira: COUCHDB-2988
2 files changed
tree: 0b506220f9b7acb3a03d1c42aee407cba8a4279c
  1. include/
  2. priv/
  3. src/
  4. test/
  5. .gitignore
  6. .travis.yml
  7. LICENSE
  8. rebar.config.script