Allow passing options to tsuru now, set GOPATH

So you can do stuff like:

```bash
$ TSURU_NOW_OPTIONS="--tsuru-from-source" vagrant up

$ vagrant ssh
vagrant@vagrant-ubuntu-trusty-64:~$ go version
go version go1.4 linux/amd64

vagrant@vagrant-ubuntu-trusty-64:~$ ls -l ~/go/src/github.com/tsuru
total 32
drwxr-xr-x  3 vagrant vagrant 4096 Dec 23 17:11 commandmocker
drwxr-xr-x  4 vagrant vagrant 4096 Dec 23 17:12 config
drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:12 docker-cluster
drwxr-xr-x 16 vagrant vagrant 4096 Dec 23 17:13 gandalf
drwxr-xr-x  3 vagrant vagrant 4096 Dec 23 17:13 go-gandalfclient
drwxr-xr-x 30 vagrant vagrant 4096 Dec 23 17:08 tsuru
drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:16 tsuru-admin
drwxr-xr-x  6 vagrant vagrant 4096 Dec 23 17:16 tsuru-client

vagrant@vagrant-ubuntu-trusty-64:~$ ls -l ~/go/bin
total 46272
-rwxr-xr-x 1 vagrant vagrant  8992488 Dec 23 17:08 godep
-rwxr-xr-x 1 vagrant vagrant 14387400 Dec 23 17:15 tsr
-rwxr-xr-x 1 vagrant vagrant 10285048 Dec 23 17:16 tsuru
-rwxr-xr-x 1 vagrant vagrant 13710040 Dec 23 17:16 tsuru-admin
```

Closes #9 

Cc: @fsouza