add .url option to constructor
Upgrade to abstract-level down: fix the _iterator setup. still a few testcases breaking
Support for options.url: Trivial error and linting
Fix limit=0
Skip the iterator snapshot test.
Cleanup
Support for url to define connection and location

Given:
```
var url = "redis://user:pass@hostname:port/dbpath";
```
One can use either of:
```
var db = levelup('dbpath', { db: redisdown, url: url });
var db = levelup(url, { db: redisdown });
```
As equivalent to:
```
var db = levelup('dbpath', { db: redisdown, host: hostname, port: port,
auth_pass: pass });
```