add Cask::SubversionDownloadStrategy

[rolandwalker]

abstract out module Cask::DownloadStrategy, add some commentary

The interface is through new url keys: :using, :trust_cert, and :revision.

There's one oddity about this implementation, which is that I tarball the directory that SVN gives us. At first glance this sounds like a horrible hack, but I've decided that it is actually a solid way to go. Here are my comments on the subject from the code:

• A single file is tractable to the rest of the Cask toolchain,
• An alternative would be to create a Directory container type. However, some type of file-serialization trick would still be needed in order to enable calculating a single checksum over a directory. So, in that alternative implementation, the special cases would propagate outside this class, including the use of tar or equivalent.
• SubversionDownloadStrategy.cached_location is not versioned
• tarball_path provides a needed return value for our overridden fetch method.
• We can also take this private opportunity to strip files from the download which are protocol-specific.

And here is an example Cask that would use this functionality in the fonts repo, addressing Homebrew/homebrew-cask-fonts#29, where multiple Casks are required to supply a single font.

class FontLekton < Cask
  url 'https://github.com/w0ng/googlefontdirectory/trunk/fonts/lekton', :using => :svn, :trust_cert => true
  homepage 'http://www.google.com/fonts/specimen/Lekton'
  version 'latest'
  no_checksum
  font 'Lekton-Bold.ttf'
  font 'Lekton-Italic.ttf'
  font 'Lekton-Regular.ttf'
end

[phinze]

reviewed and looks good 👍

solid (but not overdone) comments throughout.

eventually i think we'll probably extract out the needed functionality from uncle homebrew so we don't lean on fragile interfaces there, but this looks great for now.