Streaming log API now also honors history argument.

This addresses issue #56.

(Sorry, I misidentified the function name in [my above comment](https://github.com/soundcloud/harpoon/pull/92#issuecomment-59519641). I edited it.) 

> I'm going to beg to differ.  In function calls the name serves as documentation, and it's necessary to distinguish between the two different logs arguments.  Those things are historical logs and that's not at all apparent from just looking at the function signature unless the information is in the name.

It's not clear to me what we're differing on. Let's make it concrete. I propose this method signature:

```go
func (a *api) streamLog(history []string, l *containerLog, enc *eventsource.Encoder, stop <-chan bool)
```

My reasoning is:

- `h` is too ambiguous for a `[]string` argument
- `histLogs` or `historicalLogs` encodes stuttering