Add 'copy' download type to copy existing folders from repo

Use case -

A single distro repo contains entire drush make based project. Having a profile in the main make file current requires something like this

```
projects[example][type] = "profile"
projects[example][download][type] = "git"
projects[example][download][url] = "git@github.com:CashWilliams/distro.git"
projects[example][download][branch] = "master"
```

Which will download a 2nd copy of the repo into into the target directory. This causes an issue when trying to develop locally and making changes to the profile itself, because when running a build script (or ant) the local changes aren't yet committed.

This patch adds ability for

```
projects[example][type] = "profile"
projects[example][download][type] = "copy"
projects[example][download][url] = "file://./example"
```

Where example is a subdirectory in the repo, and make simply copies the folder as it exists into the target directory. This way local changes can be tested without committing (or pushing) them.

Right now this extends the [download] functionality to fake the download. It is working, but may make sense to change.