Fixes #2 #4 #5 #6 #7 #9

This commit fixes the following:

- handling bad HTTP status codes from the server by returning with error (like 404, 403, 500 etc; in the case of 404 the error has the properties expected with LevelUP: https://github.com/rvagg/node-levelup#get)
- custom value encoding, so that the user doesn't have to call the client functions with the `valueEncoding` everywhere:
```js
var client = require('multilevel-http').client(url, { valueEncoding: 'json' })
client.put('key', { stuff: 'here' }, cb)
```
- alias for stream methods (what you would expect with LevelUP): readStream -> createReadStream, writeStream -> createWriteStream
- added examples folder with 4 files: 1) basic example 2) http-auth using express 3) example with jsonEncoding by default 4) using writeStream
- `client.batch` now sends the options to the server
- replaced 'options' with 'meta' in README file and added extra line stating that the user should check the examples folder for advanced usage of the module

There's one slight problem:

- the test doesn't pass for the `client.writeStream` method, although if you checkout the writeStream example (from the examples folder) it works just fine, so I suspect there's something wrong with the test rather than the actual implementation. I didn't insist much on this, since 1) the method actually works 2) it will be removed from LevelUP soon

Let me know if you like this or there's something to be changed.