Printing actual http error, closing response body

Before:

```
2014/05/21 18:33:21 Post failed: Not OK
```

After:

```
2014/05/21 18:37:00 Post failed: Not OK: 500, No text specified
```

Also, response body should be closed since it is `io.ReadCloser`.

Probably you should update readme too, following files with empty lines will cause http error 500:

```
tail -F logfile | slackcat
```

will trigger error above, when:

```
tail -F logfile | grep --line-buffered -v '^\s*$' | slackcat
```

will work just fine.