added units to cactistyle function

This pull request is made in response on the issue 

https://github.com/graphite-project/graphite-web/issues/671

I will implemented a easy way to indicate units and perhaps we will improve a lot the look&Feel by adding a "unit" parameter to the cactiStile() function to aboid a "M/G/K" or "Mi/Gi/Ki" that has no sense without units.

So this graph

```
cactiStyle( "some*memory*metric","binary") 
```
![image](https://cloud.githubusercontent.com/assets/5883405/2655478/ba09f7ea-bfe6-11e3-93cb-39420f5c4ebb.png)

Can now be as follow by adding a "b" as unit parameter.

```
cactiStyle( "some*memory*metric","binary", "b" ) 
```


![image](https://cloud.githubusercontent.com/assets/5883405/2655466/a6391156-bfe6-11e3-8e2d-d6e9569bf617.png)

or 

```
cactiStyle( "some*memory*metric","binary", "bytes" ) 
```

![image](https://cloud.githubusercontent.com/assets/5883405/2655513/3a65fae2-bfe7-11e3-9d8f-2abdcb3224ef.png)