Add download support using SCP

Hey,

While working on [vagrant-notify](/fgrehm/vagrant-notify) I needed to [download a file](https://github.com/fgrehm/vagrant-notify/compare/267344485e...7adaf05571#L5L-1) from the guest machine and found out that it wasn't supported by vagrant.

Although I found out that the support is present on the master branch, I was wondering if by any chance you guys have plans to backport it to the 1.0 series.

Please let me know if you guys want me to implement the unit tests as well. I'll just need some help to get it working since I wasn't able to run the `unit_legacy` tests because of a bunch of:

```
ArgumentError: wrong number of arguments (2 for 3)
    /home/projects/oss/vagrant/lib/vagrant/box.rb:21:in `initialize'
    /home/projects/oss/vagrant/test/unit_legacy/vagrant/action/box/verify_test.rb:7:in `new'
    /home/projects/oss/vagrant/test/unit_legacy/vagrant/action/box/verify_test.rb:7:in `block in <class:VerifyBoxActionTest>'
    /home/fabio/.rbenv/versions/1.9.3-p327-patched/lib/ruby/gems/1.9.1/gems/contest-0.1.3/lib/contest.rb:22:in `instance_eval'
    /home/fabio/.rbenv/versions/1.9.3-p327-patched/lib/ruby/gems/1.9.1/gems/contest-0.1.3/lib/contest.rb:22:in `block in setup'
    /home/fabio/.rbenv/versions/1.9.3-p327-patched/lib/ruby/gems/1.9.1/gems/mocha-0.13.1/lib/mocha/integration/mini_test/version_230_to_2101.rb:34:in `run'
```

Cheers