Make chmod configurable for remote file download

If composer.phar will be downloaded the file got a hard chmod 0755.
This means only the user and the group can execute composer.
A listing on my vagrant machine:

```
vagrant@precise64:~$ cd /var/chef/cache/composer/
vagrant@precise64:/var/chef/cache/composer$ ls -al
total 952
drwxr-xr-x  2 root    root   4096 Feb  6 12:34 .
drwxr-xr-x 16 vagrant root   4096 Feb  6 12:34 ..
-rwxrwxr--  1 root    root 963742 Feb  6 12:34 composer.phar
```

If i get ssh access to my vagrant via `vagrant ssh` i am logged in with the vagrant user.
The vagrant user got no permission to execute composer:

```
vagrant@precise64:~$ /usr/local/bin/composer
-bash: /usr/local/bin/composer: Permission denied
```

This PR makes the chmod configurable. No breaking change.
What do you think about this?