label y-position for all-zero-valued graph

https://github.com/c3js/c3/blob/98769492d07b6103bfc30a0254ccb1e1ec1cca50/src/text.js

getYForText function looks to make a decision for dropping the labels below bars for negative graphs, and treats the all-zero-values case as a negative bar graph

If you are graphing data that is always non-zero positive this can cause the labels to disappear behind the x-axis, I have looked around a bit and I can't see a way to override this decision through configuration

Reproduction code

http://jsfiddle.net/jaa7e1h4/

```
// label for zero values visible above x-axis
var chart = c3.generate({
data: {
columns: [
['data1', 1, 0, 0, 0, 0, 0]
],
labels: true,
type: 'bar'
}
});
```

http://jsfiddle.net/jaa7e1h4/1/

```
// change to all zeroes, no labels visible
var chart = c3.generate({
data: {
columns: [
['data1', 0, 0, 0, 0, 0, 0]
],
labels: true,
type: 'bar'
}
});
```

C3js version c3@0.4.11
Confirmed in Chrome, Firefox