Issue title: Logs showing up more than once
 Issue body: I am setting up a logger and it is working fine expect I tried using the CLI tool and certain logs are being printed more than once

The logger I created looks like the following:

```javascript
const logger = bunyan.createLogger({
	name: 'logger',
	streams: streams: [
		{
			level: 'info',
			stream: process.stdout,
		},
		{
			level: 'warn',
			stream: process.stdout,
		},
		{
			level: 'error',
			stream: process.stderr,

		},
		{
			level: 'fatal',
			stream: process.stderr,
		},
		{
			level: 'debug',
			stream: process.stdout,
		},
	],
});
```
Then I created just a simple test script that looks like this:
```javascript
const log = require('./index');

log.info('this is some info');
log.error('this is an error');
log.debug('this is some debugging info');
log.warn('you have been waaaaarrned');
log.fatal('ohhhh shiiiiitt we are dying');
```
And this is the output I am getting:
```
{"name":"name","hostname":"Alexanders-MacBook-Pro.local","pid":11200,"level":50,"msg":"this is an error","time":"2015-12-04T21:06:34.688Z","v":0}
{"name":"name","hostname":"Alexanders-MacBook-Pro.local","pid":11200,"level":60,"msg":"ohhhh shiiiiitt we are dying","time":"2015-12-04T21:06:34.688Z","v":0}
{"name":"name","hostname":"Alexanders-MacBook-Pro.local","pid":11200,"level":60,"msg":"ohhhh shiiiiitt we are dying","time":"2015-12-04T21:06:34.688Z","v":0}
[2015-12-04T21:06:34.685Z]  INFO: name/11200 on Alexanders-MacBook-Pro.local: this is some info
[2015-12-04T21:06:34.685Z]  INFO: name/11200 on Alexanders-MacBook-Pro.local: this is some info
[2015-12-04T21:06:34.688Z] ERROR: name/11200 on Alexanders-MacBook-Pro.local: this is an error
[2015-12-04T21:06:34.688Z] ERROR: name/11200 on Alexanders-MacBook-Pro.local: this is an error
[2015-12-04T21:06:34.688Z] ERROR: name/11200 on Alexanders-MacBook-Pro.local: this is an error
[2015-12-04T21:06:34.688Z] DEBUG: name/11200 on Alexanders-MacBook-Pro.local: this is some debugging info
[2015-12-04T21:06:34.688Z]  WARN: name/11200 on Alexanders-MacBook-Pro.local: you have been waaaaarrned
[2015-12-04T21:06:34.688Z]  WARN: name/11200 on Alexanders-MacBook-Pro.local: you have been waaaaarrned
[2015-12-04T21:06:34.688Z]  WARN: name/11200 on Alexanders-MacBook-Pro.local: you have been waaaaarrned
[2015-12-04T21:06:34.688Z] FATAL: name/11200 on Alexanders-MacBook-Pro.local: ohhhh shiiiiitt we are dying
[2015-12-04T21:06:34.688Z] FATAL: name/11200 on Alexanders-MacBook-Pro.local: ohhhh shiiiiitt we are dying
[2015-12-04T21:06:34.688Z] FATAL: name/11200 on Alexanders-MacBook-Pro.local: ohhhh shiiiiitt we are dying
```

So two things I notice. Some of the JSON objects aren't getting swallowed by the pipe and some items are being output 2 or 3 times. 

Any help would be greatly appreciated
 Comments: 
 Comment 0: So I now realize that when you set the level you are creating a stream for any logs to that are at that level or more severe. I am guessing this is standard behavior in logging utilities and I made the mistake due to inexperience. That said documentation of this behavior might help avoid noob mistakes like this. I am going to submit a PR for this
 Comment 1: @aray12 Thanks for the question. Sorry for the delay in responding.

Yes, you are right that this is the intended design. You were not alone in misunderstanding this: see https://github.com/trentm/node-bunyan/issues/334   Thanks for the PR, I'll take a look when I can (I'm currently working through triaging all current bunyan issues/PRs).
{'source': 'GithubIssues', 'repo_name_with_owner': 'trentm/node-bunyan', 'issue_id': 120486840, 'repo_stars': 7433.0}