7z support.

I tend to use 7z for archival compression because it is significantly smaller than zip, especially when there are multiple files. Unfortunately, it's not natively supported by OSX.

This diff works well for me (including multiple files), but has the following issues:

- Assumes that `/usr/local/bin/7z` exists, which is not the case for a native OSX install. However, this is where `brew install p7zip` places it. I have not updated the documentation to mention this, because I'm not sure what's best.
  - The best thing for the user would be to bundle `p7zip` in the compression plugin. It's pretty small the [license is LGPLv2](http://sourceforge.net/projects/p7zip/), though I'm not sure what that means for bundling.
- `decompressFile` assumes Archive Utility can open the archive, which does not work for `7z` files.