Fractional values in histogram bins

If you pass the following histogram config
```[ { metric: 'render', bins: [ 0.01, 0.1, 1, 10, 'inf'] } ]```
statsd would send to carbon something like that
```
render.histogram.bin_0.01 value timestamp
render.histogram.bin_0.1 value timestamp
render.histogram.bin_1 value timestamp
render.histogram.bin_10 value timestamp
render.histogram.bin_inf value timestamp
```
First two metrics will make dirs in whisper database, which are undesirable for structure.

I think line 111 should be changed in process_metrics.js to
```js
bin_name = 'bin_' + bins[bin_i].replace('.', '_');
```