Need to be able to change working directory in a process

Essentially, I think procstreams be able to do the following. 

``` javascript
p('cd temp/repositories/')
    .and('git clone '+ android)
    .and('cd incubator-cordova-android')
        .and('git fetch --tags')
```

Right now it doesn't keep track of the working directory. We talked the other night about this. You were saying you were going to implement a process.chd into procstreams. 
